explain :: in c++ code example

Example 1: How to make a function in C++

//first lets create a function
/*void is for starting something, anything after void will be the name of your
function which will be followed by () */
void yourFunction() {
//your code will be here, anything here will be the code in the yourFunction
  cout << "Functions"
}
//now we have to go to our main function, the only function the compiler reads
int main() {
  myFunction(); //you call the function, the code we put in it earlier will be executed
  return 0;
}

Example 2: function declerations in C++

#include <iostream>
#include <stdlib.h>
#include <iomanip>
#include <fstream>
using namespace std;

void function_one(double, double, double);

int main() {
  double r1 = 1.0;
  double r2 = 2.0;
  double x = 0.0;
  function_one(r1, r2, x);
  return 0;
}

void function_one(double rmin, double rmax, double x0) {
  cout << "Function got called" << endl;
}