The antes is a mandatory bet in addition to the blinds. Unlike the blinds, which are paid by only two players per hand (small and big blind), the antes is paid by all players on all hands. They are mostly found in tournaments, although some card rooms also have them in games of cash game. The antes is usually 10% of the size of the big blind. On a 10-player table with blinds to 50/100's, the initial pot will be 10 (ante value) x 10 players = 100 + 50 (small blind) + 100 (big blind) so 250.
Playing strategy with antes
The cash game usually arrive late in the 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 before our very eyes.