it means whatever you want it to mean. it can be an affirmative applause, an insulting criticism, and many things in between.
"She's going to get it with that guy tonight." - She's going to hook up with him.
A teammate could shout, "Get it!" to you when you're fighting for a ball during a game.
"Oh uh, nice outfit.. get it." - a sarcastic compliment, which really means the speaker believes you look ridiculous and wonders what you were thinking that morning.