Chicken bone broth, what is the brown stuff at the bottom of the pot?
Could be a lot of things, depending on how clean the bones were, but assuming everything was clean, then my guess would be bone marrow. It's exactly that color and texture:



(Source: My Life As A Foodie)

It gets darker when cooked - refer to the link above for more photos. Don't worry about safety, bone marrow is nutritious and delicious.

You're more likely to end up with marrow in your broth if any of the bones were cut, cracked, or broken, either intentionally or during the cooking process.