פירוש על חגיגה 2:6

טבל לחולין – with the purpose of non-holy produce. And furthermore this degree [of stringency]: A person who immerses in order to consume non-holy produce and he intended to immerse for the purpose of being pure for non-holy produce.

If he immersed for unconsecrated [food], and was presumed to be fit to eat unconsecrated [food], he is prohibited from [eating second] tithe.
If he immersed for [second] tithe, and was presumed to be fit to eat [second] tithe, he is prohibited from [eating] terumah.
If he immersed for terumah, and was presumed to be fit to eat terumah, he is prohibited from [eating] holy things.
If he immersed for holy things, and was presumed to be fit to eat holy things he is prohibited from [touching the waters of] purification.
If one immersed for something possessing a stricter [degree of holiness], one is permitted [to have contact with] something possessing a lighter [degree of holiness].
If he immersed but without special intention, it is as though he had not immersed.
This mishnah deals with the intention that one has to have when one immerses. There are various levels of holy objects ranked below from lowest to highest:
1) Unconsecrated food.
2) Second tithe (eaten by its owners in Jerusalem).
3) Terumah (separated from produce and given to priests).
4) Sacrifices
5) The waters of purification made from the ashes of the red heifer.
The general rule of the mishnah is quite clear and stated explicitly at the end of the mishnah itself. If one immerses with the intention of eating a less holy thing, say unconsecrated food, he cannot count that immersion in order to eat a more holy thing. He would have to immerse again to eat the more holy thing. However, if one immerses with the intention of eating a holy thing, say a sacrifice, that immersion counts for eating a less holy thing such as terumah. Finally, if one immerses without any specific intention in mind, the immersion doesn’t count.
This explanation should aid in explaining the whole mishnah, so you will not see a fuller explanation below.

אסור למעשר – it is forbidden to eat the Second Tithe which is consumed in Jerusalem until he would ritually immerse for the sake of the [Second] Tithe, and so the case with all of them.

אסור בחטאת – in waters sanctified in the ashes of the [Red] Heifer.

ולא הוחזק – He did not intend for the purpose of a pure ritual immersion, but merely for washing.

כאילו לא טבל – for the [Second] Tithe, and all the more so for Terumah/priest’s due and for sacred food. But the immersion is for non-holy produce, for non-holy produce does not require religious intention.