Alright, so this massaging motion using the front paws is called kneading and it’s a natural and instinctive trait in felines. Not all cats knead, but it’s a common behaviour which starts at a young age and stays through adulthood. The motion is dubbed “making biscuits” because it resembles kneading dough.
Some cats knead by retracting their claws as they rhythmically alternate between the paws, while others extend them. Cats may sometimes appear to be in a kind of trance mid doing it; many purr with eyes closed, some even drool. I have experienced the drooling part after bonding with my sister's cat, though that was when I was the one massaging her as she was peacefully purring and lying on my chest.
The reason why it is believed that such massaging motion is instinctive is that newborn kittens know how to knead mama’s belly with their tiny front paws as they nurse. They do so to stimulate a flow of milk through her nipples. It starts shortly after birth and they keep doing it even when older. That is because it remains associated with the rewarding comfort of nursing — as an emotional security. Although cats separated from their mothers will still knead as adults.
No one is exactly sure why cats do that, but there are a few theories.
Some say it may be an indication that the female is willing and ready to mate, and that she’ll be going into heat (Estrus) soon. Others say it’s a sign of stress or anxiety and they do it to calm themselves.
It is also believed that some cats knead to get attention or to show contentment.
Another theory is that all cats are territorial beings and the behaviour remains from the times when cats were wild. They had used similar motions in the past to pat down grass and leaves to make beds for sleeping or for giving birth. In actual fact, there are scent glands in the soft pads on the bottom of cats’ paws, and kneading works like some kind of scent-marking to a territory or belonging.
It could also be because it's comforting for them to do it on soft and flexible surfaces. Or, simply, they may just be massaging out of love.
Now that you know the possible reasons for such behaviour, if there is kneading and the kitty-cats look happy, then let them be. You can even enjoy it with them in case they're doing it to you. But if they are overdoing it then you should probably try to find out why.
Big Cat Hybrids
Why Flamingos Are Pinkish-Orange
Animals That Are Not To Be Confused
What Nomad Lions Can Teach Us About Growing Through Life
Things I Wish All Dog Owners Would Understand