When a method is declared as abstract it is mandatory for the child class to implement it, without which it can't use other members and methods of the parent class.
We know that when method declared as virtual can be implemented in child class by creating object of parent class but it is optional when we declare it as Virtual. But in some case there might be requirement that child class must and should implement particular method, in this case it should be declare as Abstract. When method is declared as abstract it is mandatory for child class to implement it, without which it can't use other members and methods of the parent class. First it must implement the abstract method in the parent class as there can be a method body not implemented for an abstract class in a parent class.Some more points for Abstract classes
OOPs Principle and Theory