Bro is an informal, casual way to say “brother,” used mostly by boys and young men when talking to friends. You might hear someone say “Hey, bro!” as a greeting, or “Thanks, bro” to show appreciation. It's a friendly term that signals closeness, like calling someone “buddy” or “pal.”

When people call each other bro, they're usually expressing camaraderie or solidarity, even if they're not actually related. A group of close friends might call themselves bros.

Sometimes people use bro in a teasing or sarcastic way, especially when someone is acting overly casual or laid-back. You might hear someone say “Whoa, easy there, bro” to a friend who's getting too competitive during a game.

The word can also describe a certain stereotype: a bro might be someone very focused on sports, gym workouts, or acting cool. But mostly, it's just a relaxed, friendly way for people to address each other, similar to how some girls might say “girl” or “girlfriend” when talking to friends.