What does thorns do on mine craft?

Answer 1

Throns is an enchantment that you can have on certain pieces of armor including the chest piece. the effects of this enchantment are that there is a chance that when you get hit the player or mob that has hit you will take a certain amount of damage. the higher level the thorns enchantement level the more likely that the player or mob will take damage.

Answer 2

It basically means enemies who hit you take damage too. I'm not sure about the percentage or whatever but if you get hit with Thorns, the enemy gets hit too.