Definition of villain
At the poker table, any player who is not us (Hero) is considered a villain. When we talk more specifically about villains, we are talking about a player that you have played against in a particular hand. When you want to ask your friends for advice about a hand you have played, you will present the hand in question and the player whose cards are shown (you) will be called "hero" in English, and your opponent will be categorized as villain.
Reverse the roles
We can also reverse the roles for a reverse hand analysis. For example, let's say you made a bluff on the river and you want to know if the bluff is good. You could reverse the roles and pretend that the hero is the villain to find out if you made a good bluff. You could then ask your friends whether or not hero (who is actually villain) should call the villain's (who is actually hero) bet. This is a common vocabulary for understanding each other when reviewing hands.