Basically people who believe in pseudoscience about "retaining" being good for you (which is the opposite of what medical research has proved) and/or a sign of mental fortitude, nonsense like that.

To make matters worse, there's a lot of overlap with "men's rights" and other organized misogyny groups such as Proud Boys and toxic masculinity peddlers such as Andrew Tate and the like.

TL;DR: It's silly at best, and deeply problematic if not downright harmful in many cases