Let's call that "smacking". Some of mine do that when I'm holding them. I don't know for sure what it signifies, but the hens that do it are the hens that feel entitled to be picked up and held on demand. Their demand, that is. I have the feeling it's a self satisfied smacking.
These hens also "paw" the ground when standing at my feet looking up at me, which I know indicates they want to snuggle in my arms. One in particular would be content to be a "purse chicken", carried around all day with me wherever I go as some do with small dogs.