What does a "timing push" 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.

There are times where certain races are succeptable to certain tactics for brief intervals. Regardless of what they do.

For example. A fast Terran player can get a couple of marauders in an enemy base before they have time for the proper counter. It's really only a few seconds, but it's enough to cause some damage, and it may put them at a disadvantage later.

But If the defending player doesn't counter it at all, then it can be devastating.

Source [BlizzCast 14]