There's probably a number of different phrases for this particular cat trait; making biscuits has to be one of the cutest!

Anyway, what is 'making biscuits' and why do cats do it?

Whilst on your lap your cat may start to trample you with her paws, purring as she goes. She usually has a blissful look on her face.

A cat is returning to kitten-hood when she does this as she was conditioned to do this as a kitten to stimulate the flow of milk on her mum's belly. When they dribble it is in anticipation of the delicious meal she will receive.

She is, in fact, demonstrating just how much she loves and trusts you :)