Commentary for Oholot 13:5

אלו ממעטים את הטפח – a window that its measure is when it is open a handbreadth, and one of all of these that is considered here is placed in it and it reduces the airspace from a handbreadth, it interposes, that the defilement doesn’t pass to the other side.

The following [objects serve to] reduce [the area of a square] handbreadth:
[A portion] of less than an olive-size of flesh [of a corpse] reduces [the opening for uncleanness that is] caused by a quarter of a kav of bones [from a corpse];
[A portion] of less than a barley-corn size of bone reduces [the opening for uncleanness that is] caused by an olive-sized portion of flesh;
Less than an olive-sized portion of a corpse,
Less than an olive-sized portion of carrion,
Less than a lentil-sized portion of sheretz;
Less than an egg-sized portion of food;
Produce growing next to the window,
A cobweb having substance;
The carcass of a clean bird that he did not intend to eat,
And the carcass of an unclean bird that had been intended [for food] that had not been rendered susceptible [to uncleanness], or which had been rendered susceptible [to uncleanness] but had not been intended [for food].
Our mishnah deals with a window between two rooms that is a square handbreadth and therefore will allow impurity to travel between one room and the other. However, if there is placed in the window something that is not susceptible to impurity, it reduces the size of the window and blocks the impurity from traveling through. The mishnah lists some of the things that cause the window to be reduced in size. Tomorrow's mishnah will list things that do not block the impurity.
Section one: In one room there is a quarter kav of bones, an amount which causes impurity in an ohel. There is a window that is a handbreadth square that would allow the impurity to travel to the adjacent room. However, in the window there is a piece of flesh of corpse that is less than the size of an olive. In this case, the window size is considered reduced and the impurity is confined to the room in which the bones are found. However, if the impurity came from an olive sized portion of flesh and there was less than an olive sized portion of flesh in the window, the flesh in the window would be reckoned together with the flesh in the room, and the impurity would travel to the next room. In other words, like substances (flesh) join together whereas unlike substances (flesh and bone) do not.
Section two: This is the opposite scenario. In the room there is an olive-sized portion of flesh, sufficient to cause impurity in an ohel. In the window there is a barley-corn size piece of bone, which is not sufficient to cause impurity. Again, the piece of bone in the window reduces its size and the impurity doesn't go to the other room. However, in this case if there was a quarter kav of bones in the room and a small piece of bone in the window, the size of the window would not be reduced.
Section three: The less than olive-sized portion of flesh is mentioned here again because it is the opposite of the olive-sized portion of flesh which does not reduce the window, as we will learn in mishnah six.
Sections four-six: These substances do not transmit impurity. Furthermore, since they are of a different substance than bones or flesh, they reduce the size of the window no matter what is in the house: bones or flesh.
Section seven: The produce reduces the size of the window if it began to grow near the window and then spread into the window.
Section eight: For the cobweb to reduce the window it must have substance.
Section nine: The carcass of a clean bird that one did not intend to eat (even had it been slaughtered properly) does not cause impurity. Therefore, it reduces the size of the window.
Section ten: The opposite is true for the carcass of an unclean bird. It causes impurity only if one intended to eat the bird and it was rendered susceptible to impurity by coming into contact with water after it died. If he intended to eat it, or it wasn't rendered susceptible to impurity, it is not susceptible to impurity and it does reduce the size of the window.

על ידי רובע עצמות – if there is in the house a quarter [KAB] of bones that defile in a tent, and less than an olive’s bulk from the flesh of a corpse that is placed in the window, it lessens it. And specifically, less than an olive’s bulk, for if it were an olive’s bulk, it itself defiles in a tent. And specifically, through a quarter [Kab] of bones. But if there is in the house an olive’s bulk of flesh from a corpse, nothing less than this olive’s bulk can lessen it, for on the contrary, flesh combines with flesh. And that is the reason also that specifically flesh reduces through bones, but not bones through bones, for bones with bones combine.

פחות מעצם כשעורה -but a bone like [the size of] a barleycorn does not reduce/lessen the size, because it defiles through contact and through carrying.

פחות מכזית מן המת – now [the Tanna/teacher of the Mishnah] has repeated his teaching, even though he taught it in the first clause [of the Mishnah] to inform us/teach us that he lessens/reduces [the size] through all that defile in a tent, such as a quarter [LOG] of blood and a spoonful of mass of earth from a grave containing parts of a decayed human body if of a certain quantity and all of those that are taught in Chapter Two [of Tractate Ohalot].

פחות מכזית מן הנבלה וכו' – all of these that are considered from here and onward, that they reduce whether through bones or whether through flesh.

ופחות מכביצה אוכלים – that foods less than an egg’s bulk do not defile others and are not susceptible to receive ritual defilement from the Torah, therefore it reduces and interposes in the face of the defilement.

והתבואה – that took root far from the wall and its foliage straightened and closed the window, for it is attached to the ground.

וככי שיש הן ממש (the web-like marrow of reeds/substantial spiderweb in which there is substance) – spider’s web that are called the web-like marrow of reeds. But I heard that they are a species of spider’s web that is found within the reed. And sometimes it is very thin and it is web-like marrow of reeds and there is no substance to them, and sometimes it is similar to the weaving of cloth and it is like a web-like marrow of reeds that has substance to it.

ונבלת עוף טהור שלא חישב עליה – that the carcass of fowl/birds is ritually pure and doesn’t defile until one considers it/given it thought for eating, and then it defiles with the ritual impurity of foods like an egg’s bulk and does not require certification [to be susceptible to ritual uncleanness].

נבלת עוף טמא שחשב עליה ולא הכשירה – for the carrion of an impure bird/fowl requires intention/though and requires certification [to be susceptible to ritual uncleanness, that is to say, that when water comes upon it or one of the seven liquids (i.e., dew, water, wine, olive oil, blood, milk and honey – see Tractate Makhshirin, Chapter 6, Mishnah 4), but if it is lacking thought/intention or certification of [to be susceptible to ritual uncleanness], it is pure and it reduces/lessens [the handbreadth] and interposes in the face of defilement.