A max bet rule is a bonus condition that limits how much you can stake on each spin while a bonus is active or wagering is in progress. Bets above the limit can lead the casino to void the bonus and any winnings from it.
The rule is separate from the legal stake limit on GB online slots and is often lower. Some games will let you set a stake above the bonus limit, so the responsibility for staying under it usually falls on the player.
Casinos use it to stop a bonus being cleared with a few very large bets. If you take a bonus, check the max bet figure in its terms before you start, and keep the stake below it until wagering is complete or the bonus is forfeited.