Enmity, also known as threat, hate, or aggro, is a mechanic that causes the enemy monsters to target and attack a player. When multiple players are attacking an enemy, enemy A.I. will target the player with the highest enmity. Players increase their enmity by attacking, healing, applying status effects or performing specific enmity increasing actions like Provoke or your job's ranged attack. Players can also reduce their enmity by executing specific Enmity reducing actions such as Shirk.
Players can keep track of their enmity with the Threat Meter U.I. underneath the Job icons of you and your party members (by default). The number represents the enmity order of the party from highest to lowest and the letter "A" represents the player currently holding aggro, the meter represents how close the player is in enmity to the player with the number above theirs. Another Enmity U.I. is the icon next to the names of enemies currently engaged in combat on the Enemy List, there are 4 different icons going from Green to Orange to Red, if the icon is fully Red that means you are currently holding aggro.
Certain bosses have mechanics that allow them to target players that are not the highest on the enmity list. In these instances you will see a character with an "A" as well as a character with a "1". An example is the first phase of the boss fight with Livia sas Junius, where she will target the last player who used a Cannon on her, regardless of who is on top of the enmity table.
All Tank Jobs have abilities (generically called "Tank Stance(s)" by players) that drastically increase the amount of Enmity they generate, these are shown both as Buffs with an infinite duration and on the Tank's Job gauge. Since these stances are coded as buffs, they are automatically deactivated when the player is level-synced.