macroFollow

It's when you make a hot button to perform certain tasks for you. For example to cast a heal spell and give the healing message with one touch.

To give you the long answer:

if you go to the last tab of your actions box (looks like a little voice bubble from comics) and click one of the arrows left or right, you can see some blank pages for macros. If you right click on one you will get a pop up box. You can give it a name for example

"Heal"

Then in the first one you can give the command to cast your heal spell, which assuming its the first spell in your list would be /cast 1

You can then alert your group that you are healing a player, using %T to say who's getting healed.

So if you do

/g Casting minor heal on %T, please don't move far.
/cast 1

you can then place this in your hot keys so you can just hit say 4, to tell the group your healing, and then cast the spell.

There is also the "bad" kind of macro, which uses third party programs, basically what it does is creates something similar to as above, but with much more complex actions, usually in the form of actually doing tradeskills, killing mobs, moving from place to place, etc. These types of macros are against EULA to use, but some people try to get away with it anyways.