大家帮我看看这个程序是什么意思，我很是看不明白哎

//参考程序2
#include<iostream.h>
#include<math.h>
const double PI=3.14159;
float area(float a=3,double b=4,double c=5);
int main()
{        double s;
    s=area();    
cout<<"s="<<s<<endl;    
s=area(6);    
cout<<"s="<<s<<endl;    
s=area(2,4);    
cout<<"s="<<s<<endl;    
s=area(5,6,7);    
cout<<"s="<<s<<endl;    
return 0;
}
float area(float a,double b,double c)
{    float s,S;    
         s=1/2.0*(a+b+c);    
        S=sqrt(s*(s-a)*(s-b)*(s-c));    
         return S;
}

函数area是求三角形面积的，三个参数是边长