Fuel reserve is like having a small extra snack in your backpack when you go on a hike in case you get hungry. Just like how you might not finish your whole snack, a plane may not use all of its fuel during a flight.
So, the airplane has special tanks that hold fuel for the journey, but the pilots also make sure to bring a little extra fuel just in case. That extra fuel is called the fuel reserve. It's sort of like a "just in case" amount, in case something unexpected comes up during the flight, like bad weather or a longer flight than originally planned.
The amount of fuel reserve a plane carries depends on many factors like the size of the airplane, the length of the flight, and the weather conditions. Pilots have to be careful to make sure that they have enough fuel reserve to make it through any unexpected circumstances, and even have rules and regulations they must follow to ensure they always have enough fuel, just in case.