Deep Breath......
Mechanical bounce refers to the cycling of the marker causing the trigger to 'bounce' off your finger or if set too light to fire by itself. The more metal etc that moves when you fire the marker and the lighter the trigger pull the more likely you are to see mech bounce. Cockers have a rep for it due to a larger reciprocating mass and light magnetic triggers, the least bouncy are probably Matrixes/Shockers.
Electronic bounce refers to the fact that when a switch is closed and opened you get a lot of little signals as well as one big one, and some boards read the little signals and thus fire more times than you pull the trigger. Debounce allows you to set the length of signal that the board registers as a pull, low for lots of shots and getting kicked out of events, high for slower ROF but safer and more legal. Truth be told, almost all electronic markers add a little unless you set the debounce quite high, at which point everyone complains their marker is slower than their fingers