It means you be lying. So like if you say something like I killed somebody yesterday and said later that you was fucking a hoe, someone you told that bullshit to, will say you be cappin
You: my side bitches Bianca and shonda'laquesha was getting fucked yesterday and this other side hoe showed up and I shot her.
Jon Jon: nigga you be cappin now stop lying before I take yo side bitches and yo momz
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.
FRIEND A: "Did you just take a stealthie of me?"
FRIEND B (turning phone around): "no I was just using snapchat's new filter, see?"