It means that they have been scratched or grooved and have to be turned on a lathe to give them a completely flat surface again.
Its almost cheaper to buy new rotors than to have them turned corey. This is because the rotors warp easily or you wore your brake pads down to the. Rivets which tore up the rotor
Turning the rotors, resurfacing the rotors, and machining the rotors are all the same thing. It puts a new surface on the rotors by taking the metal off with a brake lathe. You should always resurface the rotors or replace them when doing the brake pads. Even if the rotors are not grooved they will have a build up or glazing on the rotors that will decrease the stopping power in emergency situations or cause heat build up that will cause the rotor to warp. The inexpensive rotors made in China are not bad, I've installed them on many cars with out a problem and it's almost the same price as resurfacing the old rotors. It's also something you can replace on your own with out needing a machine shop.