Definition of square
A four of a kind hand is when a player gets four cards of the same denomination (for example 5-5-5-5). Four of a kind is the hand needed to beat a full hand. To beat a four of a kind hand, we need to make a straight flush (a straight with the same suit, for example 5-6-7-8-9 all in hearts). When two players have four of a kind, the highest four of a kind wins the pot (8-8-8-8 will beat 7-7-7-7). It should be noted that the frequency of getting a four of a kind is very low.
Square and badbeat jackpot
Some poker rooms offer a badbeat jackpot, which is a progressive jackpot that is hit when a square is beaten. Depending on the room, the height of the four of a kind will vary. It is so rare in poker to lose with a four of a kind that the badbeat jackpot will make your bad luck a good luck if it happens. In general, you'll find that the lowest qualifying 4-of-a-kind hand is a pair of 8's. So if you make a pair of 8's in your hand and lose the hand to a stronger hand, you'll get 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 cash amount can vary depending on how often this jackpot is won, usually between 25,000 Euros and 1 million Euros.