If you find lumps on your cats neck or anywhere else, it could be cysts, or tumors.
Cysts are not a problem unless they grow too big and not all tumors are cancer.
Sometimes cysts are caused by other cat bites if your cat has been in a fight. I once had a cat that had a huge cyst on his backside because of a fight and it even needed a tube to drain it.
Taking your cat to the veterinarian is wise as this way you will know what the problem is.
The Vet might order some tests but might have an idea what the problem is by just palpating the area.