C++ STL - How does the third parameter in fill() in the below program work?

Question

In STL, the syntax for Fill is template void fill (ForwardIterator first, ForwardIterator last, const T& val);

The third parameter in the below program should be int. What does it imply by the statement operator int() const? Can we have int as a function name? How does the return value of this function get converted to int and stored in a vector(As in third parameter of fill function)?

#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;
void print(int v) {
    cout << v << ", ";
}

struct Sequence {
    int start;
    Sequence(int a,int b): start(a) { }
    Sequence(int start) :start(start) {}
     operator int() const{
        return start;//LINE I
    }
};
int main() {
    vector<int> v1(7);
    fill(v1.begin(), v1.end(), Sequence(1,2));//LINE II
    for_each(v1.begin(), v1.end(), print);
    return 0;
}