The maximum hit of a monster is the highest amount of damage that a monster will inflict for a given attack style. If a player is hit by this monster, the player will receive damage up to and including this amount. The amount of damage done is not affected by a players defence level. There are certain ways to reduce the amount of damage through protection, such as prayer.
The lunar spell "Monster Examine" may be used to identify the max melee hit of a monster. If a monster doesn't use melee attacks, and only attacks with ranged or magic (such as Dark Wizards), then it will display the monster's max hit with the attack style it uses. Monsters with more than one attack style (ex. melee and magic), may have two different maximum hit levels, one for each attack style. This information is useful for players that can protect from a certain style of attack with prayer.
Monsters have a certain fighting style too, making them more or less effective versus what you are wearing.
Currently, the highest monster max hit is 251, from TzKal-Zuk, a boss found at the final wave of The Inferno.