Macro options are a way to control actions based on various pieces of information. To dive right into an example, the following macro will cast Renew on a friendly target and Shadow Word: Pain on a hostile one.
/cast [help] Renew; [harm] Shadow Word: PainWhen you run this macro, the [help] condition is checked. This determines whether your target is someone you can cast beneficial spells on. If the [help] is true, it then casts Renew and the macro moves to the next line. Otherwise (either you have no target, or you can't cast a helpful spell on your target), it falls through to the next clause. Now it checks for the [harm] condition. [harm] is just like [help] but for offensive spells. If true, it casts Shadow Word: Pain. If it isn't true (no target or you can't harm your target) then it does nothing because there are no more clauses.

Note: I could have left the [harm] check out and it would have functioned in much the same way. However, if you have no target or your target can be neither helped nor harmed, you would receive an error message or, depending on the spell, the target selector cursor.