An extra ante paid only by the button, on top of everyone's single ante.
The common Short Deck format: the button's extra ante of 1 takes the place of the preflop bet. Example: at a 6-handed table the pot starts at 5 × 1 + 2 = 7, and the others call (limp) the extra 1 or raise. Action starts left of the button on every street and the button acts last. With no raise, the button can check.