If you're familiar with Java, it's like the "extends" keyword.
Inheritance allows you to make a class that's based off another class known as the base class; a "Cat" class could inherit from a "Mammal" class (the Mammal class is the base of the Cat class), which would inherit from an "Animal" class, and so forth.
A class can access the protected and public members of its base class, but not the private members.