Aggro is a term derived from the word "aggravate" that originated in MMORPGs. It represents how much a non-playing character/enemy wants to target and/or attack a playing character.
In Army of Two, the amount of aggro for each player is visible on the Aggrometer. The player with the most aggro glows red, while his partner, with low aggro, appears transparent. Weapons can affect how much aggro you have. For example, shiny gold or chrome weapons will increase your aggro, while using a suppressor will decrease it.
Using and controlling aggro is a strategic aspect of the game. For example, one player can intentionally increase his aggro so that the enemies focus on him. Meanwhile his partner, with low aggro, can move around unnoticed, allowing him to sneak behind the enemies and sabotage them.