Are my cats fighting or playing?

I have a cat who I have had for over a year and is around a year and a half old. We just adopted a kitten about 2 weeks ago who is 5 weeks old. We kept them separated and slowly introduced them to each other. They are with each other more now. I do notice the older cat pinning the younger cat and biting her neck. The kitten growls and occasionally hisses but the older cat lets her up and she casually walks away and stays in the same area, sometimes jumping into the cat tree 2 feet away or staying in the cat tree at the bottom. I can t tell if the kitten is hiding or not but she doesn t run and bolt from him and I have seen her pounce on his tail a couple times. I just want to make sure this is harmless playing and not fighting. Can any one help answer this for me and even better, if they are fighting some steps to help them have a healthy relationship