The term basically means what it says. It is where you do not do any that will be unfair to your opponent, meaning no cheating or winning unfairly in any way possible. A great battle is always won without cheating or doing anything that are against the rules of the game.
Fighting fair is essentially a term that means no cheap shots. No blindside force, no shots to the privates, no hair pulling, eye gouging, etc. Different people interpret it differently.
