An ante bet is an optional extra charge on each spin, often a fixed percentage of the stake, that increases how often the bonus round triggers. Some games call it double chance or a bet multiplier.
It does not make the game cheaper per feature in any reliable way; it changes the shape of play. You pay more per spin, the bonus comes more often, and the game's RTP with the ante active is shown separately in the paytable, sometimes slightly different from the standard figure.
Where an ante bet is offered, the extra amount is part of your total bet for that game cycle and counts towards the GB online stake limits. Whether a GB version of a game offers an ante bet depends on the game. It is distinct from a bonus buy, which pays to skip straight to the feature and is not offered on GB-licensed sites.