Friend keyword is used for friend functions.
In C++ class has data member and member function. Data member declare private that cannot be accessed outside the class, means non-member function of class. But some time situation is arises, private data member need to access outside the class. That is done by friend function. A friend function is called non-member function, when we create friend function, we can access private member of class.
friend void disp(); //friend function
obj.x=5; //access the variable
cout<<"the value of x="<<obj.x; //print the value
disp(); //calling function
In this example we create a disp(),which is private. But the function is friend means we can access outside the class. Friend function access the class variable x by using dot operator.By using friend functions ,the member function of one class can be accessed by another class.