[Help] means helpful character, like a player or NPC. /cast [target=target, help] Heal( ) would only cast heal if your target was a helpful player / NPC. If you have no target or a enemy target, this macro would do nothing.

you mentioned being in combat, that would look like this:
[Combat] mean you are currently in combat. /cast [target=target,combat] Spell ( ) would only cast the spell if you're in combat. If you aren't in combat, this macro will do nothing.