PLEASE explain to me how self employment taxes work like I'm five years old.

It says if you make under $400 as a self employed person you don't have to pay taxes. But under $400 what. 400 a month? 400 a year? Why the fuck is this not mandatory learning in school? I'm 20 and I feel like I should be in a corner with the dunce hat because I don't know fucking anything about how money+being adult works.