What does AFK mean

A timing push involves making an attack at a certain point in your build order, such that the attack coincides with the completion of a researched upgrade, or a certain mass of units.

For example: you might see a Terran Bio timing push, where the Terran player moves his forces out such that Stimpack technology will have just finished as they reach the enemy forces.

Also applies to Weapon / Armor upgrades. Since the Weapon / Armor upgrades often completely cancel each other out, it's best to start fights when you have the upgrade, and your opponent does not. If you wait too long, your opponent (if they know what they're doing) will complete their own counter-research, and your advantage will be lost.

Hence, timing push; Timing an attack such that it occurs when you have the momentary advantage.

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.