Some of my kitties do that. I think each cat has a different meaning for it and it is up to us to figure it out.
From Marmalade, it means he wants your attention and would you please pet him. Marm will make a little 'cry' sound, maybe paw at your leg or arm and when you look at him, he will hold a small 'pinch' of skin while looking in your eyes and purring. From Dusty, it is usually when I am petting him and he will grab my wrist/hand with his front paws and gently 'hold' me to tell my hand to stay and pet him. Shadow will sort of give me little nibbles when she is in the throes of enjoying being petted. She walks in circles, wags her tail, rears up to catch her nose in your palm and rubs her face into your hand, giving the little nibbles. If Malibu gently grabs me with her teeth, it means she wants me to stop petting her, and when I do stop, she will 'soften' her eyes, loosen her hold and give me a few licks where she held me.