As far as I know it’s fat.

That’s assuming it’s good quality (ie organic, pasture raised, grass fed, etc) animal products you used.

If it’s not then usually the chemicals and harmful stuff binds to fat easier than muscle, so the film on top could be tainted.

That being said, I:

1. Leave a little in my broth so when I want it up I have that bit of extra flavor the fat provided

2. I freeze/refrigerate the rest and use it as a I would butter or coconut oil anytime I’m cooking eggs or vegetables wherein I want a little extra flavor…or simply as a non-stick agent