Okay kiddo, let me explain fire regime to you. A "fire regime" is like the schedule of how often and how intense fires happen in a certain area. It's like if you knew there was always a dessert night on Fridays, and sometimes it was cake and sometimes it was ice cream - that would be the dessert regime.
For example, some forests might have really big, intense fires every ten years, while other forests might have smaller, more frequent fires every few years. Sometimes people call these different types of fire regimes "high severity" or "low severity."
Knowing the fire regime of a forest or other area can help scientists and forest managers figure out how to take care of it. If they know fires happen every few years, they might not need to worry as much about putting them out. But if they know fires only happen once in a while, they might need to do something to make sure that when a fire does happen, it doesn't get too out of control.
Does that make sense, kiddo?