What is default arguments in cpp

What is default arguments in cpp

PROGRAM:

#include<iostream>
using namespace std;
class interest{
    float p,r,n;
    public:
    void setdata( ){
    cout<<"\n P: ";
    cin>>p;
    cout<<" R: ";
    cin>>r;
    cout<<" N: ";
    cin>>n;
    calc(p,n,r);
    calc(p,n);//!uses default arguments
    }
    float calc(float p,float n,float r=0.25){ //!default argument
    cout<<"\n\t\t\t Interset is : "<<(p*r*n);
    }
};

int main(){
    interest i;
    i.setdata();
return 0;
}