An abstract class in C++ is a class that has at least one pure virtual function. A pure virtual function is a function that has no definition and is declared using the "= 0" syntax. Abstract classes are designed to be used as base classes, and their descendants must define the pure virtual function. Otherwise, the subclass would become an abstract class itself.

Abstract classes are used to represent general concepts that can be used as base classes for concrete classes. They are essential to providing an abstraction to the code to make it reusable and extendable. For example, a Vehicle parent class with Truck and Motorbike inheriting from it is an abstraction that easily allows more vehicles to be added. However, even though all vehicles have wheels, not all vehicles have the same number of wheels – this is where a pure virtual function is needed.

In summary, an abstract class in C++ is a class that has at least one pure virtual function and is designed to be used as a base class. Its descendants must define the pure virtual function, and abstract classes are essential to providing an abstraction to the code to make it reusable and extendable.