2000 Jaguar S-Type Cats system Fault

I have a Jaguar S-Type that has the Cats system fault message that comes up when i start the car every morning. when the light is on the cars drives pretty ruff. after driving it for a lil while the traction control light comes on. then i turn off the car for about a minute or two and start the engine again and the car starts normally n everything works fine. i want to know y is that n what does the cats system fault light mean.