A subclass is something that extends the functionality of your existing class. I.e.
Superclass - describes the catagory of objects:
public abstract class Fruit {
public abstract Color color;
}
Subclass1 - describes attributes of the individual Fruit objects:
public class Apple extends Fruit {
Color color = red;
}
Subclass2 - describes attributes of the individual Fruit objects:
public class Banana extends Fruit {
Color color = yellow;
}
The 'abstract' keyword in the superclass means that the class will only define the mandatory information that each subclass must have i.e. A piece of fruit must have a color so it is defines in the super class and all subclasses must 'inherit' that attribute and define the value that describes the specific object.
Does that make sense?