Mishnah 6 in chapter 3 of Berachos deals with the relationship between the immersion of a baal keri, which is no more than an ordinance of Ezra - the ordinance that one may not learn Torah or pray while he is a baal keri - and forms of tumah that come from the Torah itself. The question is how these two matters interact with one another.
The Mishnah also teaches the concept of a poletes shichvas zera, which is, as it were, the female counterpart of a baal keri. A woman herself has no semen, but if after marital relations semen is discharged from within her body - even if she has already immersed in a mikvah after relations - she too receives the status of a baal keri and may not pray.
The three cases in the Mishnah:
"Zav she'ra'ah keri" - a zav does not discharge semen; rather, this is a type of illness in which a man has certain discharges at certain times and a certain number of times, after which he is considered a zav and is obligated in immersion by Torah law. The question concerns one who was a zav and afterward had a seminal emission, so that he became a baal keri as well: must he immerse on account of the keri? After all, a zav requires seven clean days free of discharge, whereas a baal keri may immerse immediately. And if he will remain a zav in any case, is there any point in his immersing because he is a baal keri?
"V'niddah shepaltah shichvas zera" - a niddah is obligated by Torah law to immerse in a mikvah. And although a niddah by Torah law does not require seven clean days (which apply only to a zavah), nevertheless once she has seen blood even a single time she is tamei for seven days. If in the middle of those days she discharged semen from relations she had before she became a niddah, she would go to the mikvah because she is a poletes shichvas zera, but she remains a niddah, for this immersion has no effect whatsoever on her niddah status.
"V'hameshameshes shera'asah niddah" - a woman who had marital relations, and because she came into contact with semen is obligated to immerse in a mikvah like any baal keri, and afterward became a niddah. Must she immerse because she had relations or because she is a poletes shichvas zera?
The dispute of the Tannaim:
The Tanna Kamma says: "Tzerichin tevilah" - they all require immersion because of Ezra's ordinance, in order to pray and to learn Torah, even though this immersion accomplishes nothing for the tumah of a zav or the tumah of a niddah, upon which it has no effect at all. In any event, the very fact that he is a baal keri, or that she is a poletes shichvas zera, obligates immersion.
"V'Rabbi Yehudah poter" - in Rabbi Yehudah's view, since the immersion has no effect on the tumah that comes from the Torah, he or she does not immerse even on account of being a baal keri.
In summary: in this Mishnah we learned the relationship between Ezra's ordinance regarding a baal keri and forms of tumah that come from the Torah, and we clarified the concept of a poletes shichvas zera. Three cases were discussed: a zav who had a seminal emission, a niddah who discharged semen, and a woman who had relations and then became a niddah. According to the Tanna Kamma they all require immersion because of the ordinance, even though it accomplishes nothing for the tumah from the Torah, while Rabbi Yehudah exempts them.