The base class library is like a huge toy chest full of different toys for a computer to play with. These toys are called "classes" and they help the computer do different things. For example, there might be a toy car class that helps the computer make a game about racing cars.
The base class library comes with lots of pre-made toys that the computer can use right away, without having to make them from scratch. These pre-made toys are called "base classes."
Just like how you might use different toys from your toy chest to build a big city or a fort, computer programmers use different base classes from the base class library to build software programs.
By using base classes, programmers don't have to start from scratch every time they make a new program. They can just use the pre-made toys from the base class library and build on top of them to make their program do what they want it to do.