#showtooltip -- Shows the tooltip for Shackle Undead
/clearfocus [modifier:shift] -- clears your focus if you hold shift
/clearfocus [button:2] -- also clears your focus if you right click the button on your hotbar.
/clearfocus [target=focus,dead] -- clears if the focus target is dead
/clearfocus [target=focus,noexists] -- clears focus if focus target doesn't exist
/focus [target=focus,noexists] -- sets your current target as a focus target, if you don't have a focus target
/cast [target=focus] Shackle Undead -- casts Shackle on your focus target

So basically, if you don't have a focus the macro sets it for you on the first button press and on the 2nd it casts Shackle. If you hit it while your target is either dead or doesn't exist, it will clear it and then set the new one. If you don't have something targeted, it will just clear it.

Isn't that 'Sets your current target as a focus target, if you don't have a focus target?'