Overload is a gameplay mechanic used in some missions. All attacks by Bloons will fire streams of 10 attacks (firing 1 every 0.1 seconds) instead of 1. This only applies to projectiles, and doesn't apply to passive attacks. Once you defeat the O.V.E.R.L.O.A.D., it becomes a toggleable mode outside of missions. Beating any level (includes missions) in Overload mode is the only way to get Overload Shards.