Definition of a bad beat
A bad beat is when a player loses a hand when he was favored. The term could be translated as "taking a hard hit" or "taking a bad beat". For example, you have a pair of aces on the turn. Your opponent has a pair of kings. The board is A-5-7-K. You both go all-in. At this point, you know that you have a little over 97% chance of winning the pot. The river falls and it's another king. Your opponent wins the pot with four of a kind. You will say that you had a bad beat.
What is a bad beat jackpot?
Some online poker rooms offer their players a bad beat jackpot option. At these particular tables (identified as such), in addition to the usual rake, an additional rake will be taken from each pot and sent directly to a progressive jackpot. If a player loses a hand with a straight 8 or better (the nature of the hand may vary by poker room) with two cards in hand (i.e. two 8's), the player who lost the hand will receive 50% of the prize pool, the player who won the hand will receive 25% of the prize pool and the remaining 25% will be split equally among all the other players at the table.