Fortnite Mechanics?

Could someone explain to me exactly what mechanics are and could they also tell me how I can test whether mine are good or bad and how I can improve it if mine are not so good?

Mechanics means how good you are at building, aiming, moving, etc. But it doesn't mean that you play smart, but that you can theoretically play well. You can test this by doing an edit course.

I'm actually good at editing so now I have good mechanics when I'm good at building?

In principle, yes.