Many pet foods use common proteins: chicken, duck, beef, turkey. Novel proteins are proteins that are not typically found in pet food: rabbit, pork, pheasant, veison, even kangaroo among others
I'm not sure where you read that novel proteins should be avoided. Typically a cat who has a allergy or senitivity to a common protein like chicken will be put on a "limited ingredient diet" that has a novel protein. The thought is that because the novel protein is less commonly found in pet food, there ie less of a chance that the cat would also have similar allergy or senitivity to a novel protein.
Some people like to rotate novel proteins with common proteins anyways. It keeps the cat from getting bored with the food and some people feel that it lesses the chance of an allergy or sensitivity from developing in the first place.