Shield break
Each character has a shield with a particular size and strength. With each hit inflicted within a period of time, the shield will diminish until it is broken. This results in the character receiving damage, being thrown into the air, and being stunned for a few moments.
Shield breaking moves are ones that can be comboed to prevent a character from rolling out or from dropping the shield without being hit. The easiest shield breaking move is Kirby's up tilt, which will break a shield in 3-4 hits without the shielder being able to roll out. Marth, Roy, and Ike's neutral-B attacks, at a full charge, will break a shield, period.
One fact that fighters who use Jigglypuff should be careful about is its shield breaking. Like other characters, if Jigglypuff's shield breaks, Jigglypuff will fly upward and then be stunned for a short period of time. Jigglypuff, however, flies several hundred times farther upward than anyother character in the game(s). This is well enough to kill Jigglypuff, unless there is a ceiling or some other obstacle preventing it from flying straight up.