Okay kiddo, have you ever had a spare toy in case something happened to your favorite one? That's just like a flight spare! For a very important machine like an airplane, sometimes there is an extra part called a "flight spare" kept just in case one of the parts breaks or stops working.
It's like having an extra cookie just in case you drop the first one on the ground! The flight spare can be used to fix the airplane quickly, without having to wait for a new part to be made or shipped.
Companies who make airplanes know how important it is to have backups, just like how you know it's important to have a backup stuffed animal if your favorite one gets too dirty or torn. And once the broken part is fixed with the flight spare, the airplane can fly safely again!