Question

What does an antagonist usually do ​

Answer 1

An antagonist typically wrecks havoc on the protagonist. For example (short and sweet):

Voldemort is the antagonist in Harry Potter.

Answer 2

The antagonist is normally the villain of the story, like in Cinderella, you could say that her step mother was the antagonist. The antagonist will do anything in their power to make the life of the protagonist/main character a horrible one.