Mouseover macros in WoW explained like I'm five?

On retail you might not even need macros any more. In Options > Gameplay > Combat there's a Mouseover Cast setting. Turn it on and every spell on your bars goes to whoever you're hovering. Macros still make sense if you only want it on two or three spells.

normally a spell goes to whoever you've clicked on. a mouseover macro casts it on whoever your mouse is hovering over instead, so you can heal someone without changing your target. /cast [@mouseover,help,nodead][] Flash Heal that reads as: if my mouse is on a living friendly, heal them. if not, do the normal thing. swap the spell name for yours, put the macro on your bar and use it like the normal button.