Commentary for Mikvaot 2:7

לנגבן – The Aramaic translation of (Genesis 9:13): “the waters began to dry [from the earth],” for they had certainly dried, and specifically to dry them, that we did not intend for them to receive the water, therefore, they (i.e., the water) would not be made “drawn,” as it is written (Leviticus 11:36): “However, a spring or cistern [in which water is collected shall be pure,]” just as a spring does not have the grasp of the hand by a human, even a well/cistern that does not have the grasp of the hand by a human.

If one had left wine-jars on the roof to dry and they became filled with water:
Rabbi Eliezer says: if it was the season of rain and there was [in the cistern] a little water, one may break the jars; otherwise one may not break them.
Rabbi Joshua says: in either case one may break them or tilt them over, but one may not empty [them into the cistern].
This mishnah deals with rain water that collected in jars on the roof. Is this rain water considered to be "drawn water" such that it will disqualify the water in the cistern to be used as a mikveh.
Section one: The person left the jars on the roof to dry out so that he could use them for wine. Had he left them there with the purpose of gathering water the collected water would count as drawn water and could not be put into the mikveh.
Another complicating factor is that it's the rainy season so it looks like he might have left the jars up there intentionally to gather water (note that if it's not the rainy season, this scenario simply won't occur in Israel, when it's not the rainy season, it doesn't rain at all).
According to Rabbi Eliezer, if there's a little water in the cistern already, then he may break the jars and let the water flow into the cistern and the mikveh will be valid. This is because the water in the jars is not really drawn water, it just looks like drawn water. Therefore, there is room to be lenient when there is already some water in the cistern. However, if there is no water in the cistern, then he may not do so. As he was in mishnah four, Rabbi Eliezer is stringent when it comes to adding water to a cistern at the outset.
Section three: Rabbi Joshua is more lenient. First of all, there is no difference between letting the water flow in before there is other water there and after there is other water there. In both cases, one may either break the jar or even tilt it over and let the water flow into the cistern. The one thing he may not do is pick up the jars and pour them into the mikveh. Picking up the jars turns the water into drawn water which invalidates a mikveh, unless there are already in there 40 seahs of valid water.

אם עונת גשמים היא – that the heavens did not become knotted with clouds and that rain fell.

אם יש בו כמעט מים בבור – we have this reading [of "אם"/if], and we do not have the reading of "או"/or.

ישבר – the jars/cannisters [should be broken] and the water that is in them will fall to the cistern/pit. That this is the conducting of the water through a channel, since that within broken utensils, they (i.e., the water) fall to the cistern/pit.

ואם לאו – that it is not the rainy season, alternatively, that there is no water in the pit/cistern.

לא ישבר – for if it is not the rainy season, he would not be able to make a Mikveh/ritual bath from the water that is in the jars/cannisters alone, for Rabbi Eliezer holds that drawn waters, all of which or most of which were conducted through a channel is invalids. But when it is the rainy season, he would break it to channel these waters, and most of the Mikveh would be filled up from rain water. But when there isn’t any water in it (i.e., the Mikveh) at all, he should not break it, even though it is the rainy season, lest there fall from the water a quarter of a LOG into the pit/cistern prior to breaking it. But Rabbi Eliezer, according to his reasoning, who stated above (see Tractate Mikvaot, Chapter 2, Mishnah 4), that a quarter of a LOG of water ab initio invalidates the Mikveh/ritual bath.

ר' יהושע אומר בין כך ובין כך ישבר – that he (i.e., Rabbi Yehoshua) holds that three LOGS [of drawn water] invalidate the Mikveh/ritual bath whether at the beginning or whether at the end, and not one-quarter of a LOG, and we do not suspect that perhaps three LOGS [of drawn water] will fall into the Mikveh/ritual bath prior to breaking [of the jar/cannister]. But even though it is not the rainy season, he should break it, for it is able to continue until [it reaches] the measurement of the Mikveh, for he holds that drawn [water] that continues throughout, is ritually pure. But the Halakha is not according to Rabbi Eliezer.

או יכפה (or turn it upside down) – that he turns the jars/cannisters that are on the roof upside down so that that the water doesn’t fall from inside them [directly] into the cistern/pit, but rather, that they should fall to one side, and from there continue to the pit/cistern.

אבל לא יערה – that if he raised/lifted up the jars/cannister and emptied them, they (i.e., the waters) would become like drawn water, for they have the grasp of the hand of a human.