= You're nothing.
However, 'you ain't shit' isn't necessarily an insult. You can comfort someone by saying that they are not shit.
You ain't shit vs. You are shit is definitely not the same. 'Ain't' is a general negation vernacular term.

the whale tail is the shape formed when a g-string rides up high over a womans trousers

you just trashy,ain't nothing but a bum,creep,loser,coward,waste,etc
or someone who can't do nothin,but eating,playing and sleeping

you really suck at a sport or game or anything you do