Originally, aggro was the term used to identify that mobs would attack you without you doing anything to them to provoke an attack, i.e., mere propinquity was all that was needed. If you were in an area with mosters that aggro you, and you started running, you could obtain one hell of a spectacular “train;” similarly, if you stood still, and the mobs were wandering around, they would slowly aggregate around you as they got close enough to you to trigger their attack function. Sure way to die if you were in the wrong area.
If you are in a party of people, it is now common in most games to conflate the concept with the “hate” concept, that is, the factor that causes a mob to focus its energy of attack upon a particular member of the party. Thus, if you are attacking a group of mobs, and they all are drawn to a particular member of the party, that member of the party will be said to have aggro. As is often the case with the computer era, terminology undergoes relatively swift change in meaning.