Stops an enemy from moving. Bosses can still shoot projectiles when stunned, it simply stops their movement. When an enemy is stunned they get become yellow/goldish for the duration of the stun: Stun scales with the Duration stat, the more duration you have, the longer it can stun an enemy. For how long an enemy becomes stunned (before duration gets applied) depends on the source of where it came from. For instance, Thunder Mitts always stuns for 3.0s. But if your duration multiplier is 2.0x, it will instead stun for 6.0s.