Cats and some other animals have a third eyelid, also known as the nictating membrane. It provides extra protection to the eye, produces part of the tear film and plays a role in the immune system. Normally it is retracted into the corner of the eye closest to the nose. Sometimes it can protrude and this is generally a sign of ill health or loss of condition. It may also protrude if the cat is very happy or relaxed. If your cat's third eyelid is protruded unusually it may be a sign of sickness. I would monitor them closely, if you have any concerns call your vet.