Definition of quad
A quad is when a player gets four cards of the same denomination (e.g. 5-5-5). A quad is the hand needed to beat a full hand. To beat a quad, we need to make a straight flush (a straight with the same suit, for example 5-6-7-8-9 all heart). When two players have a quad, the highest quad wins the pot (8-8-8 will beat 7-7-7). Note that the frequency of getting a quad is very low.
Quad and badbeat jackpot
Some poker rooms offer a badbeat jackpot, which is a progressive jackpot that is hit when a quad is beaten. Depending on the room, the height of the quad will vary. It's so rare in poker to lose with a quad that the badbeat jackpot will make your bad luck into good luck if it ever happens. In general, you'll find that the lowest quad to qualify is the quad of 8, so if you make a quad of 8 with a pair of 8's in hand and lose the hand to a stronger hand, you'll hit 50% of the badbeat jackpot. The player who won the hand will receive 25% of that amount and the remaining 25% will be divided among the players at the table. The amount of money can vary depending on how often this jackpot is won, usually between €25,000 and €1 million.