For the past two mitzvos, we have been discussing the zav, a man who has a discharge from the male organ that is not semen. Now we are speaking of a man who has a regular seminal discharge. A man who had such an emission would have to immerse in a mikvah; he would remain ritually unclean until nightfall.
The reason for this mitzvah is that seminal emissions are brought about by one’s physical desires. We don’t want people to preoccupy themselves with lustful thoughts; rather, they should occupy themselves with spiritual pursuits. Accordingly, the Torah responds to this expression of an obsession with one’s physicality by requiring him to spend a day in ritual impurity and, hopefully, contemplation.
This mitzvah applies in all times and places, but it has no practical application in the absence of the Temple. It is discussed in the Mishna in the tractate of Zavim and in the Talmud in tractate Shabbos on page 86a. This mitzvah is codified in the Mishneh Torah in the fifth chapter of Hilchos Sh’ar Avos HaTumah and is #105 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos.