If you have a perfectly normal (non-pathological) lack of confidence then pretending to be confident may allegedly help you. A cynic could remark that that requires knowing what that looks like and being able to pull it off, while almost nobody offering that particular "advice" seems to feel a huge need to elaborate on it.
It's a cliché.
I don't see it either, but I'm sure the .ml gentleperson is only too psyched to explain.