Cats share with humans the tendency to get kidney and bladder stones—uroliths is the technical term and they are not pleasant. Male cats are more prone to them (ditto for male humans), and they seem to be more common in cats who are fed an exclusively dry food diet. The overall condition of stones forming in the urinary tract is called feline urological syndrome (FUL).
A cat whose urinary tract is blocked by a large stone can be in intense pain, and no wonder, since it needs to urinate but can’t because of the stone blocking the path. A vet’s aid is definitely called for, and quickly. Once the stone is removed or passed, the cat’s diet has to be altered and medication given to prevent more stones from forming.