This will be his impurity through an emission… (Leviticus 15:3)
A zav was a man who had an emission from the male organ that wasn’t semen. Rather, this emission was a liquid that has been compared to the runny part of a rotten egg. Also, unlike a seminal emission, a man received no physical pleasure through such a discharge. Apparently, a person would have no difficulty distinguishing between a zav flow and a seminal emission.
The first mishna in the tractate of Zavim clarifies that a man needed to experience two such emissions to be fully rendered a zav. When a man experienced two such flows in a day, he became ritually impure and was capable of transmitting impurity to others. If the man had a third such discharge, he became obligated in a sacrifice, as we shall see in the next mitzvah.
The reason for this mitzvah is to encourage moderation in a man. The Talmud in Kiddushin (2b) tells us that emissions are brought about by excesses in eating and drinking. Being rendered unclean – and being shunned by others who do not wish to contract this uncleanliness – is meant to ensure that a person exercises some restraint, rather than partying out of control. This also goes towards explaining why a single emission does not fully render a man a zav; a single discharge is not necessarily the sign of a problem and nobody’s perfect all the time.
This mitzvah applies in all times and places, but in the absence of the Temple and the observance of most forms of ritual impurity, it has no practical application. The zav is subject of the Mishnaic tractate of Zavim; in the Talmud it is discussed in tractate Megillah (8a-b). This mitzvah of the zav is codified in the Mishneh Torah in Hilchos Metamei Mishkav U’Moshav. It is #104 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos.