It's a poker (card game) term.
The "ante" is what all players pay to participate in the game. Each player pays the ante (which is also known as "the stake") so that there is something to win even before anyone gets any cards. Depending on the game and who you are playing with, the ante can be really small (like $1 or $2 if you're playing with friends just to drink beer and smoke cigars) or quite large ($5000) to keep out the riff-raff.
When you raise the ante, you raise the minimum prize (the stakes) of the game.
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