C++ What does return function do after if statements in loop?-Prime Test

#include <iostream>

#include <cmath>

using std::cin;

using std::cout;

using std::endl;

int prime(int n){//primetest function

`int i;`
`if(n == 1) return 0;//zero- not prime`
`if(n == 2) return 1;//one- prime`
`for(i= 2; i <= ceil(sqrt(n)); i++){`
    `if(n % i == 0) return 0; //remainder is zero}`
`return 1;}`

int main(){

`unsigned long n;`
`while(cin >> n){`
    `cout << prime(n) << endl;}`

`return 0;}`

// I know that 0 means not prime and 1 means prime. Can someone please explain how are all the return functions used to accomplish this? Why not use cout << "0" for not prime and cout << "1" for prime?

Thank you. I am new to C++