A class is derive from a class which is derive from another class is known as multilevel inheritance or we can say derived class work as a base class for another derived class called multilevel inheritance.
The best example is Say we have 3 class
ClassA,ClassB and ClassC.
 
ClassB is derived from ClassA and ClassC is derived ClassB  this is multi level inheritance.
class A{
    public:
       void show(){
                  cout<<"show\n";
       }
};
class B: public A     
{
       public:
            void display(){
                        cout<<"display\n";
            }
};
class C: public B{
        public:
              void test(){
                      cout<<"test\n";
              }
};
void main(){
            C obj;
            obj.display();
            obj.show();
            obj.test();
}