As has been mentioned already, it was probably caused by a rock or some other material. There is a radial groove in the pads that can fill up with mud - happened to me and I chewed up a rotor - make sure it is clean.
How serious it is depends on how deep it is and how wide. If you haven't felt it, it may not be as bad as it looks. You should be able to find some pics of good vs bad rotors on the Web to give you some idea of what you are dealing with.
Go to a brake shop that you trust and ask for their opinion. If it's a minor scratch, they should be able to turn the disc and get rid of it. If it's too deep/wide to repair, I'm sure they will be glad to sell you new rotors/pads.
To echo what's been said above, it might be time to get new rotors/pads all around - the brake shop should be able to help with that decision. Upgrade if wanted.