This is primarily used by young African-Americans, however it is used by all American youths. To "be trippin'" (the "g" in the suffix "-ing" is usually not pronounced) can mean either that 1) someone is out of control; or that 2) they have a misconception.
Ex for use 1) "Calm down. Stop trippin' about how they got your order wrong at McDonald's."
Ex for use 2) "You're wrong about how I make poor grades in school. I make good grades in school; don't be trippin'."
There need not be an "-ing" suffix either, so that the term can be used in this way as well: "Don't trip." It is never said that someone "tripped"; only that someone was "trippin'."