An ante is a forced contribution that every player at the table posts before the cards are dealt. Because the pot already holds money when the hand starts, players have a reason to compete for it instead of waiting for premium cards.
Antes and blinds are different things. Blinds are paid by only two players, and the obligation rotates with the dealer button; an ante is paid by everyone, every hand, at a fixed amount. Some games use both, and tournaments often add antes in their later stages. In casino poker games played against the house, such as three-card variants, the ante is the opening wager that gets the player a hand.
The everyday phrase “up the ante” comes from this rule: raising the ante makes every pot larger without changing the blinds. Online, ante amounts are set per table or per blind level and should be visible in the lobby before a player sits down.