A monster in poker is a very strong hand that is almost impossible to beat. While a monster hand can be outdrawn on a later street, this is very unlikely to happen in most cases.
Monster Poker Example:
If you have an AcKc hand and the flop reveals 9c 6c 2c, you’ve flopped a monster hand in the form of an ace-high flush. The only way you can still lose with this hand is if your opponent makes a straight flush.