What is the meaning of lawful order?

An unlawful order is one that is clearly outside the authority of the person giving it.

Ordering someone to commit a crime is clearly unlawful. Killing a civilian (meaning an unarmed non-combatant, not simply someone who doesn't wear a uniform; an armed civilian is a combatant and thus a legal target) would be an example.

Countermanding an order of a superior officer is one that occurs, but is not necessarily unlawful, it can get into sticky situations.