Max win is the most a slot can pay out on a single game round, normally expressed as a multiple of the stake. A 5,000x max win, as an illustrative example, means a £1 spin can return at most £5,000.
Many modern games have a hard cap: when the total reaches it, the feature ends and pays the capped amount. Others state a theoretical maximum that comes from the maths model rather than a cut-off.
The max win says nothing about how likely it is. Top wins are typically so rare that most players will never see one. A high max win usually signals higher volatility, because the game has to fund that rare outcome by paying less elsewhere.
Max win is per round, so it depends on your stake. In Great Britain online slot stakes are capped (see stake limits), which caps the cash value of any given multiple. Our game pages show each title's max win from data.