What is the object slicing?

In Inheritance, the attributes of the base class get carried to the derived class. However, we can assign a base class with the derived class without having the contents of the derived that are uncommon between then, copied to the base class. 

Class B
{
    public:
    int i;
};

class D : public B
{
     public:
     int j;
};

int main()
{
     B B1;
     D D1;
     B1 = D1; //only i is copied to B1
}