I cooled chicken broth , there were yellow fat on top and the clear jelly underneath. what's the jelly?

the yellow is chicken fat the clear jelly is your broth. All broth cooked from meat sources with bones, etc gets like that. It is the collagen from bones and connective tissue that does that. Same way Jello is made, only fruit flavored. If your gel like material is very thick, that means your broth is very concentrated. you get the same thing in the bottom of the pan after roasting a turkey and cooling it, or a pot roast with bone in or baking a bone in ham. Once you heat it, it is liquid broth again. It is absolutely what you want for great flavor when making soup or gravy or any meat sauces.