Ok I Recently got into my book and i am up to functions and how they work and i am a bit confused maybe someone can explain better then my book is or use a better example.....

Code:
//sqrt.cpp -- use a square root function
#include <iostream>
#include <cmath>
using namespace std;
int main()
{
     double cover;   //use double for real numbers

     cout << "How many square feet of sheets do you have?" << endl;
     cin >> cover;
     double side;   //create another variable
     side = sqrt(cover); // call function, assign return value
     cout << "You can cover a square side of " << side;
     cout << " feet\nwith your sheets.\n";
     cin.get();
     cin.get();
     return 0;
}
im not grasping how this works exactly......