While you have the Stunned condition, you experience the following effects.
Inactive. You can’t take any action, Bonus Action, or Reaction.
No Concentration. Your Concentration is broken.
Speechless. You can’t speak.
Surprised. If you’re Stunned when you roll Initiative, you have Disadvantage on the roll.
Saving Throws Affected. You automatically fail Strength and Dexterity saving throws.
Attacks Affected. Attack rolls against you have Advantage.
A character can also choose to break this effect at the start of their turn by taking 4 (1d8) psychic damage per two character levels.
Source: Lazy DM’s Companion (damage to break stun)