I see people mentioning cooldowns and CD, often in reference to tanking. I have no idea what this word means - can someone explain it?

A cooldown is a period of time when an ability cannot be used.

 

You'll notice that you have several abilities that can't be used for 15 seconds or so. The boxes on your action bar turn dark for 15 seconds after you use the ability. There are many different lengths of cooldowns for various moves ranging from 3 seconds and up (the longest cooldown in the game is your fleet pass, clocking in at 12 hours with no legacy upgrade).

 

Another type of cooldown is called the global cooldown (GCD). This refers to a 1.5 second period of time after you activate an ability where all (with a very few exceptions) abilities are locked. You can see this by looking at your action bar after you activate an ability. You'll notice that all the boxes (with a few exceptions) will turn dark for just about a second.

 

You probably have gotten used to the GCD without even realizing why because just hitting buttons too fast doesn't work.

Thanks for taking the time to clear that up for me!

If you go into your preferences menu and locate "Cooldown Text" ( i think it's under user interface...not sure) it'll make it easier to understand. this displays text over the ability telling you exactly how long you have until the ability becomes available again.