A bad beat in poker is a term used to describe a situation in which a player loses a hand despite being a big favorite when all the chips go into the pot.
There are no hard and fast rules as to what constitutes a bad beat, but the bigger the advantage the player had to begin with, the worse the bad beat is considered to be.
An example of a nasty bad beat is losing to a player who has just a top pair against your middle set. For example, you flop a set of 8s on board of A-8-3 and get it all in against AK. The turn comes an A, and the river comes a K, giving your opponent a bigger full house. On the flop, you were around 98% favorite to win the hand, and you ended up losing.
As annoying as they can be, bad beats are a part of poker, which is why serious poker players don’t like listening to bad beat stories.