You’ll encounter the term hero in poker hand histories and it simply points out to the person from whose point of view the hand history is being recounted. It is the opposite of a villain.
For example, you could say something like, two players limp for $200, hero raises to $1,000, and the button 3-bets to $3,500. The limpers fold and the hero makes the call.
By using the term hero in this description, we are making it clear who we are focusing on in this hand history and whose decisions we’re primarily interested in. It’s not the limpers or the button but the hero.
It is a bit of a strange term to use in poker, but it has become commonplace in the online realm. It is now used by different poker HUDs, on poker forums, and in casual conversations.