Prefold is a rectangular shape of cotton, bamboo, or hemp fabric with layers sewn together.   When you see 2x4x2 in the description, that means this has 2 layers of fabric with 4 layers in the middle (in other words, 2 layers plus 2 more in the middle.)   You can fold them and lay them in the cover (or all-in-two's cover).  Some people like to use Snappi to secure them before placing the cover on the baby.  Prefold diaper requires a cover.