AC obviously enough refers to canceling the animation of an ability or another action. Some believe that canceling the animation of an ability lets you cast the next ability because you made your character animation-free and thus open for the next one. This is false because animations are entirely and solely visual. These grant no benefit in combat, and either if you AC your ability or not, you can't bypass the GCD.

GCD means the one second timer that makes your character free and open for an action. You can only cast one ability per second, shoot one light attack per second. There are more GCDs but the info about them will be covered further below. GCD is hidden ingame and works as a background mechanic. Your abilities will not have the loading icon like in other games.

Even though it doesn't matter if you AC your abilities or not because they abide by GCD, i will list the possible ACs:
casting another ability (some abilities have the animation longer than one second), light or heavy attacking, dodge rolling, activating block, weapon swapping, falling impact (don't move while falling so the spell is casted shortly before impact), sheathing your weapon (lol), activating a synergy, being the victim of an action that modifies your character animation (like being stunned) or a pve mechanic.

The two main GCDs are your active abilities/spells and the light/heavy attacks. Utilizing those two GCDs together makes the difference, called weaving.
There are four GCDs. Aside from the two main ones, there are also bashing and synergies. You can use three bashes in one GCD, and one synergy per second (but each ability's synergy has its own cooldown per player). You can activate two separate synergies at one second distance one from the other, but the same synergy will be available again after its own cooldown.
There is also the blocking GCD, but this is not important to be mentioned because you don't have control over it (it refers to the times your resources drain while blocking and taking damage).

AC does not have any practical effect. Either you AC or not your ability, you can only fire one ability per second thanks to GCD. If you AC an ability, visually it will not appear as being casted or it will have its effects shown earlier, but the actual effects of the ability will be given based on its timer.
For example BoL (Breath of Life, templar morph heal skill) will give its heal at the same time after being initiated either if you AC it or not. The visual effects can be canceled or shown earlier than the ability heals, but the actual healing will occur after the ability's timer rather than when you cancel its animation. The same goes for any ability, so you can't deal the damage sooner or heal the target sooner because you AC it.

GCD has a very big importance on optimizing your dps and aptitude in general. The more empty time you leave between your GCD triggers, the worse your dps will be. Overlapping GCDs known as weaving have a big importance on your overall skill and separates a weak player from a good one.
Out of the 4(+1) GCDs you can use whatever you need. The weaving is important to maintain. Bashing only when necessary. You can cast a light attack and an ability, and if necessary hold block for the rest of the second in this GCD. You can use a synergy now in this GCD, or leave it for the next good moment (for example not using a resource synergy if your resources are full).

You AC your light attack the moment you use an ability. So you trigger your first GCD (light attacks) and at the same time you trigger the second GCD (abilities). The light attack will take place simultaneously with the spell, thus not shown because the character started the spell over it. This way you use both GCDs but one of them will not have its animation showed. Even with animation shown or not, GCD cannot be bypassed, so the myth about AC raising your dps is fake. You raise your dps because you utilize both GCDs simultaneously, called weaving.

In conclusion do not care about AC, it is just a visual consequence of your character because you do two actions at the same time while your character being able to visually show only one.

But do care about GCD! You can use in a single second all the four GCDs, even tho it's a bit extreme. You only need the two main GCDs, but you can also use a synergy in that second. You can also bash in this single second if it's a case where you need to interrupt your target.

So, weaving is all you mainly need from GCDs.

video with showing how to do it and how it works, on my youtube here
https://youtu.be/R3dR5rG-Pmo

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