Aggro is the attention of an enemy, meaning which player or character the enemy chooses to attack. Managing aggro means controlling that choice so durable characters take hits and fragile characters can work safely.
Most enemies pick targets with a threat system running underneath the fight. Damage dealt, healing done, proximity, and scripted rules all feed a hidden score, and the enemy attacks whoever leads that score. Tanks are built to win it on purpose, with taunt abilities that force an enemy to face them for a set time even when someone else has dealt more damage.
Aggro matters because enemy damage is rarely survivable for everyone. If a healer pulls aggro by restoring a large amount of health at the wrong moment, the boss may turn, land one hit, and end the attempt. Good groups therefore pace their opening seconds, let the tank establish threat first, and use aggro dropping tools such as stealth, feign effects, or brief invulnerability when a pull goes wrong.
Outside group games the word is looser. In shooters and action games players say they have aggro when enemies are simply focused on them, and to aggro a room means to trigger enemies into combat, sometimes deliberately to gather them together and sometimes by accident by stepping one pace too far forward.
In play"Wait two seconds before you open fire so the tank can hold aggro."