Toggles auto-attacking of the player's current target.
This is actually a toggle. If not currently attacking, it will initiate attack. If currently attacking, it will stop attacking.
You can test your current attack "Action slot" using IsCurrentAction(actionSlot) for status (you'll have to find the auto-attack slot, though).
If you need a way to always engage auto-attack, rather than toggle it on or off, one workaround is AssistUnit("player") this will always attack if you have "Attack on assist" checked in the Advanced tab of the Interface Options panel. Note that you cannot combine this with TargetNearestEnemy() in the same function/macro: the "assist" target isn't updated fast enough.
The macro "/startattack" will always initiate auto-attack (or auto-shot for hunters, if the appropriate interface option is active).