What is the small black iron pot in photography and what is its function?

Is it from the camera or the photo? Facing the camera, I only think of the hood, which is used to eliminate stray light. For example, if you shoot in front of you at night, you may get unwanted light from the side, which is used to cover this up. Another purpose is to make your lens look longer (for X). Black spots are noise, so just reduce the sensitivity.