Threat Tables change how a mob tracks its targets. Normally Minecraft mobs do not follow any kind of rigid system of what to target - they will simply bounce back and forth between attacking whatever hits them. Threat Tables change that.
With Threat Tables enabled, mobs will keep track of how much damage each player does to them, and will attack the player that deals the most damage. That way you can avoid scenarios where one player will hit a mob and then run away while others follow behind smacking it forever, trivializing it.
Threat Tables come with several built-in features to make the mob's targeting intelligent, and use rules found in common MMORPGs. Players gain threat by dealing damage, and will lose threat if they kite the boss, stay outside of the boss' MaxCombatRange, or stay out of line of sight for long periods of time. Players will also drop threat if they leave the world or log off.
Mobs will only switch targets if another player passes 110% threat over the current target.
Note that activated threat tables will “override” the AI target selectors you specified for the mob. A mob with activated threat tables will attempt to attack any entity that deals damage to it - even if such entites are not listed in the AI target selectors or even if the AI target selector list for the mob has been swiped clean and the mob doesn't naturally attack anything or anyone.
Enabling Threat Tables
Turning on Threat Tables for a mob is easy. Just add Modules.ThreatTable: true to your mob, like this:
BigScaryBoss: Type: zombie Display: '&6Zombie' Health: 20000 Modules: ThreatTable: true
Manipulating Threat Levels
If a mob has threat tables enabled, it will always target the entity with the highest threat level on its own threat table. This process is fully automated and based on which entity does how much damage to the mob. Naturally, the entity (usually a player) dealing the most damage, will gain the most threat and become the target of the mob.
However if you would like to manually make your mob target specific entites, or just throw in some tweaks that make your mobs targeting even smarter, you can do so using the Threat Mechanic.
Threat Tables also come with an API, including a “taunt” method and threat altering methods, if another plugin author ever wanted to have skills or abilities that interact with threat.