An extra forced bet paid by everyone, or by the BB for the whole table.
It is a forced bet paid into the pot before the hand, separate from the blinds, and since the pot starts bigger it is a reason to play wider. Example. In a 6-handed game with an ante of 1, the pot is 6 before the deal. With a BB ante the BB pays for everyone at once, which is common in tournaments, and Stud games have no blinds, so everyone antes.