Trigger Bounce is when a trigger is set so damned lightly (hair-trigger) that the actual gun cycling resets the trigger, and actually makes the guns' electronics think a second trigger pull has occured, which sends another shot out of the marker, even though you've only pulled the trigger once. So you get more than one shot per trigger-pull.
If you get more than one shot per trigger pull, then your marker is bouncing.
First off, what marker do you have? Each one can be altered mechanically, or electronically to bounce, and do various other things.
How to avoid it? Dont put a hair-trigger on your marker.
I think my definitions are accurate, but if I'm wrong, correct me.