What is the meaning of Aggro?

Aggressive behaviour; loud, intimidating behaviour that convincingly threatens violence without necessarily actually becoming violent.

A measure of how belligerent a player is – a high value may inspire either avoidance or preemptive hostile action from enemies.

Hostile attention from an enemy that should target players with better defenses.

Liable to attack without being attacked first (said of monsters).

Designed for aggressive play that seeks to defeat the opponent in the early stages of the game. Favoring such decks.

To become aggressive towards the player's character.

To cause (a non-player character) to become aggressive towards the player's character.

Source: wiktionary.org

See also