What does "Rar" mean?
See the Pasuk there.
Why does it say both "Rar Besaro Es Zovo" and "Hechtim Besaro"?
Rashi: The former means that the emission comes out in the form of a liquid, the latter, even if it comes out thick and stops up the hole of the Amah.
Moshav Zekenim, from R. Nasan (in Nidah 43b): "Hechtim Besaro" teaches that there is a Shi'ur for Zov, i.e. enough to stop up the hole of the Amah. 1
Moshav Zekenim: This is unlike Chachamim in the Mishnah, who say that any amount is Metamei.