Why do cats knead?

You've guessed it, it's another sign of contentment. "A kitten instinctually kneads its mother’s teat to help stimulate her to produce milk, so kneading resembles a happy, safe and rewarding time," explains Anita.

And, like dogs, cats knead their paws onto surfaces to activate the scent glands inside their paws to mark an area as their territory. So take it to mean that they're comfortable with their surroundings. 

source: https://www.goodhousekeeping.com/