What is a fist?

Could you please explain to me in simple terms what a "fist" is? I've heard the term used in various contexts, but I'm not entirely clear on its meaning. Is it a physical object, or is it a more abstract concept? If it's a physical object, how is it formed and what are its key characteristics? Additionally, could you give me some examples of situations where the term "fist" might be used in conversation? Thank you for your help in clarifying this concept for me.