Antes are mandatory bets that are added to the blinds. Unlike blinds, which are only paid by two players per hand (small and big blinds), antes are paid by all players in all hands. Antes are most common in tournaments, although some card rooms also have them in cash games. Antes are usually 10% of the size of the big blind. So on a 10 player table with 50/100 blinds, the initial pot would be 10 (ante value) x 10 players = 100 + 50 (small blind) + 100 (big blind) so 250.
Game strategy with antes
Antes usually come in late in tournaments. At each stage, the blinds increase and in the later stages of the tournament, the structure adds antes that will be paid by all players at the table. As the antes will increase the size of the starting pot and as there will now be no more free hands, more hands will have to be played more aggressively. Failure to play enough hands will result in our stack melting away.