Subclassing is just a subclass of another class and it inherits behavior and members of its parent class.
For example, you may have a "car" class which has properties that are common to all cars such color, tires, engine, etc. It also has behaviors that are common to all cars such as drive forward, drive backwards, stop, etc.
Now for some reason you need to differentiate between different types of cars but you don't want to repeat all those properties and behaviors in each new class so you write subclasses of the "car" class.
So you may have subclasses such as "manual cars", "automatic cars", "convertibles", "electric cars", etc. All of these subclasses have some common traits inherited from the "car" class but will also have some unique characteristics that can be included in the subclass.