Okay kiddo, have you ever played with fire before? You know how it's hot and can burn things right? Well, a long time ago, people believed that there was a powerful being who controlled fire and they called him the fire god.
This fire god was believed to be very powerful and sometimes people would even offer him sacrifices, like food or even animals, to keep him happy and not let him cause any harm.
People used to pray to the fire god to keep them warm in cold weather and to help them cook their food. They also believed that the fire god had the power to bring light into their homes during dark nights.
Some cultures had different names and beliefs for the fire god. For example, in Greek mythology there was a god named Hephaestus who was responsible for the fire and the forge, while in Hinduism there is a goddess named Agni who is the personification of fire.
So, in short, a fire god was a mythical being that ancient people believed had power over fire and they worshiped him to keep themselves safe and warm.