Kneading is a common behavior among cats that is believed to have originated from their kittenhood. The cat pushes in and out with its front paws on soft surfaces, such as blankets, laps or other animals. The main reason for kneading is during nursing, where the kitten treads its paws on the mother’s body to stimulate the flow of milk. The motion of kneading or treading their paws probably makes it a pleasurable activity, according to Susan Hazel, an associate professor of animal behavior, welfare and ethics at the University of Adelaide in Australia. Another plausible explanation for kneading might be depositing scents on the paw pads to mark territories.
https://www.livescience.com/animals/cats/why-do-cats-knead