nodead means you won't target a target that is already dead
Mouseover means it will target what your mouse icon is currently on when you press the macro
Harm is basically enemy targets
noexists means no current target is selected
So in a nutshell the macro will the following when activated;
Dismount you from a mount IF your mounted OR take you out of Ghost wolf IF your in Ghost wolf.
Select the enemy target your mouse icon is currently over and target it IF the target is actually a enemy AND is NOT a dead enemy body on the ground.
IF you did NOT have a enemy target under your mouse icon WHEN you pressed this macro, it will target the nearest enemy standing...assuming your in range to target anything.
Once you have aquired a target (either by mouseover or otherwise) it will put you into attack mode (aka "right clicked"), and launch the name of the ability that you inputted below the startattack line (ex: lightning bolt, Chain Lightning, Frost Shock, etc)
All this is done in ONE stroke of the macro!
Pretty nifty if you think about it. Saves you a bunch of time trying to fumble around the screen unclicking a mount, hitting a seperate attack button, finding the spell you want, etc...
Thus all my LB, CL, ES, SS spells ALL have at least thoughs few lines in the macro, granted I do have more elaborate macro structures to make more things even easier...but thats my little gamer secret.
You should be studying www.wowwiki.com for macro creation and game mechanics, most of your concerns should be answered there.