On the eighth day, he shall take two pigeons or two doves… (Leviticus 15: 14)
As mentioned in the previous mitzvah, two non-seminal discharges from the male organ rendered a man a zav and a third obligated him in a sacrifice.
At the end of a week, a zav would go to the mikvah and immerse, after which he would be purified. On the following day, those who were so obligated would bring a sacrifice consisting of two birds, one of which would be offered as a sin offering, while the other was offered as a burnt offering.
As mentioned in the previous mitzvah, the reason for the laws of the zav is to encourage moderation in eating and drinking as it was overindulgence that brought about such discharges. As with the metzora (Mitzvah #176), the sacrifice is meant to humble the zav so that he re-thinks his ways.
This mitzvah only applied in Temple times. It is discussed in the Mishna in the tractate of Zavim and in the Talmud in tractate Megillah on pages 8a-b. This mitzvah is codified in the Mishneh Torah in the first chapter of Hilchos Mechusarei Kaparah and is #74 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos.