The phrase is most often used at the end of a statement you want to have acknowledged or you want to drive home. The statement may or may not have been said by the person who will then say, "Catch it!"
Definition #2: It can also be used to confirm a statement or a non verbal action such as a look.
Person a): "Listen to this new Madonna you will love it."
Person b): "It sounds hot! I really like it."
Person a): "Catch it!"
**You catch a friend looking at a your new Gucci belt that you wore for the first time.
Person wearing Gucci belt: "Catch it!"
Person a): "you are acting like a bitch tonight!"
Person b): "Catch it!"