A bad beat means that a player lose with a very good hand very unlucky
Many players think if they lose a All-In with AK vs A6 that this a bad beat. In reality it is not a bad beat. A real bad beat is if your opponent has a chance of max 5-10% to win with his hand, or if he hit two perfect cards to beat you.
If a player hold for example QJ and the Flop shows QJJ. Your opponent has TT and push All-In. The Turn and the River will bring TT. That would be a real bad beat.