I have heard "aggro" said in commentaries in a few different contexts and I have no idea what it really means. I can figure out what the commentator means, but I don't understand that word.
Aggro as a term originated in MMO's, but its usage has spread considerably.
Originally it was coined to describe any creature who would attack you on sight. So an "aggro" mob was one who would attack without being provoked, as opposed to one who wouldn't attack unless you attacked it first. The related usages here are "aggro range" or "aggro radius" which is the distance at which the mob will attack, and "aggro chain" which is whether or not the mob will bring his friends along, even if they are outside of normal aggro range.
As the games evolved "aggro" became the state of being attacked. If you were being attacked, you "had aggro", and if you did too much damage to something and it started attacking you instead of someone else you "stole aggro". If you did something stupid you could have "too much aggro" which meant too many things were drawn to attack you.
In your example, he's saying that the unit AI can be exploited because they don't chain aggro, so you can move into their aggro range, and individually "pull" them from their groups and eliminate them one at a time.
It means that you move closer to enemy forces (may be slightly attack 'em) and move back.
If your opponent does not react properly then only couple of units will "aggro" and start moving to attack your units. So you can easily pick them alone.
i.e. as terran you can aggro enemy to go into range of your tanks
you can also check wiki. Starcraft's AI uses distance and type of unit to determine what to attack.
It is a shortened form of aggressive. It describes monsters that will recklessly attack with no regards to their own defence. This term is in Magic the Gathering, a popular TCG, it is a type of deck with cheap, fast creatures that have high attack but low defence to try kill the opponent as quickly as possible. Aggro creatures are mainly in red. Mono-Red decks and Red-White decks are commonly used as aggro decks.
"Aggro" means to have aggression, and to be extremely violent. Monsters with high "aggro" are to avoided unless you want a serious fight on your hands. I never heard this term until I played video games, don't know where it came from, all sources seem to be inaccurate when they say it was before MMORPG's. It clearly means violent, mad, and aggressive in MMO terms though, which makes sense anyway, any other usage is silly. Which is why no one else has ever heard of it before MMORPG's because it IS a term that originated from them, at least in an intelligent form anyway.
Aggro means to draw aggressions of others towards yourself; obtaining surrounding enemy attention and weapons fire towards onto oneself. An example would be an army unit in combat, exchanging fire, and you need to get the rest of your unit to safer position. Multiple people are "pinned down" (cant move at all or they will take a hit. Therefore, you (have slightly better position) start firing at enemies. You thereby become the Aggro so your people can move. Used in the US army and military.