What is my cat doing on my other cat?

Um i'm a little confused, I'm home on a sick day and my mum told me to feed the cats. When I went to grab some food for them I saw Sammy (one of my cats) on top of Tilda (my other cat). and Sammy was like moving up and down on Tilda but Tilda didn't move our try to scratch him or anything, is he like cleaning his fur or something?