Maximum bet amounts are the largest stakes that a punter can bet on any given market. Maximum payout amounts are the max amount a punter can win from any one bet. Bookmakers use maximum bet & payout amounts as a form of insurance against making large losses.
A MaxBet refers to a maximum bet, which is essentially a limit set by an online casino that governs how much you can bet on online casino games. This restriction appears on jackpot slots most often, but it applies to all other games, too.