Machshirin, Chapter 4, Mishnah 9. Produce becomes able to receive tumah only once it has been wet with water that came upon it with the owner's approval, beratzon. Our Mishnah asks a practical question of doubt: water was deliberately drawn through an irrigation channel, and afterwards produce passed through it. How long do we assume that the channel was still damp enough to render that produce muchshar?
"Hamemalei bekilon, ad shloshah yamim, temein." A person drew water through a kilon, the irrigation pipe or channel used for watering, and the water certainly went through it beratzon. For three days afterward, the Mishnah rules, the produce is "tamei." As always in this tractate, "tamei" here does not mean that the produce is actually tamei; it means that it has undergone hechsher and is now capable of receiving tumah should something tamei come into contact with it.
The situation the Tanna is addressing is one of doubt. Produce passed through the channel after the water had been run through it, and we do not know whether it actually got wet. For a period of three days we must assume that it did: we work on the assumption that the channel was still damp and had not yet dried out. After three days, we assume the channel is already dry, and the produce that went through it did not receive hechsher.
Rabbi Akiva draws the lines
The Mishnah continues with a second voice: "Rabbi Akiva omer". He is not disputing the first ruling but defining its boundaries. Where the facts are known, they decide the case. Suppose the channel "nagvu", it dried up right after the water was drawn, perhaps because the weather was warm, perhaps because nothing was left standing in it. Then produce that passes through afterward is "miyad tehorin", tahor from that moment, meaning no hechsher took place at all. Once we can see the reality in front of us, there is nothing left for a presumption of three days to accomplish. And in the opposite situation, where the pipe is "lo nagvu", the produce remains temein even "ad shloshim yom" later.
And in the reverse case, "ve'im lo nagvu": if the channel did not dry out, then even up to thirty days later the produce is "tamei," for as long as the dampness lasts it continues to effect hechsher lekabel tumah.
So the three day period of the first Tanna is meant for the uncertain case, where it cannot be established whether moisture was still in the channel when the produce went through. Rabbi Akiva's contribution is that established facts override the presumption in both directions: if we know the channel was dry, no hechsher ever occurred, and if we know it was still wet, hechsher continues throughout the entire time that the moisture remains.