From Brotato Wiki
+8 Ranged Damage
+8 Elemental Damage
Your max HP is capped at its current value
Handcuffs is an in-game item that can be bought from Shop or can be found in a Crate. Most Items increase the Stats of the player while some of them provide unique effects.
Handcuffs is a unique item. Unique items can only be bought or found once per run.
Handcuffs provide a large amount of damage for a small cost. But it comes with the downside of locking your Max HP when you buy it. For example if you buy handcuffs while you have 50 Max HP, your Max HP can never be higher than 50.
However you can still buy items that lowers your Max HP and it can cause you to have less than 50 Max HP. The lock is only any upper bound, it doesn't prevent you from getting lower.
Handcuffs can be a powerful pickup late in the game if you've already gotten the Max HP you wanted for the wave 20 boss fight.