What does this code mean? please ,would you give some hints for the code below?

Kindly, I need some hints to understand this code below, which is a solution for this problem >> [https://open.kattis.com/problems/racingalphabet][1]

#include <cmath>
#include <iomanip>
#include <iostream>

using namespace std;

int main() {
    int n;
    string s;
    int a[128];
    a[' '] = 0;
    a['\''] = 1;
    for (int i = 0; i < 26; i++) a['A' + i] = i + 2;
    cin >> n;
    getline(cin, s);
    while (n--) {
        getline(cin, s);
        int c = 0;
        for (int i = 0; i < s.size() - 1; i++) {
            int d = (a[s[i]] - a[s[i + 1]]) % 28;
            if (d < 0) d += 28;
            if (d > 14) d = 28 - d;
            c += d;
        }
        cout << setprecision(10) << s.size() + (c * 4 * M_PI / 28) << endl;
    }
}


  [1]: https://open.kattis.com/problems/racingalphabet