Need explanation of a code in c

I am completely new to embedded programming, I'm examining the code below and trying to understand how it work, but I really got stuck.

#include <bits/stdc++.h>

using namespace std;

const int N = 1005;

int n, m, q;

int g[N][N], dp[N][N], med[N][N];

int rows, cols, need, query[N*N], size;

int search(int L, int R, int query[], int size) {

    if (L == R) return L;

    int M = (L + R) / 2;

    for (int i = 0; i < n; ++i)

        for (int j = 0; j < m; ++j)

            dp[i+1][j+1] = -dp[i][j]+dp[i+1][j]+dp[i][j+1]+int(g[i][j] < M);


    int tsize = 0;

    for (int k = 0; k < size; ++k) {

        int i = query[k] >> 16;

        int j = query[k] & ((1 << 16) - 1);

        int c = dp[i][j]-dp[i-rows][j]-dp[i][j-cols]+dp[i-rows][j-cols];


        if (c <= need) {


            query[tsize++] = query[k];

        }

    }


    if (tsize > 0)

        return search(M+1, R, query, tsize);

    else

        return search(L, M, query, size);

}

int main() {

    scanf("%d%d%d", &n, &m, &q);

    int mx = 0;

    for (int i = 0; i < n; ++i)

        for (int j = 0; j < m; ++j) {

            scanf("%d", &g[i][j]);

            mx = max(mx, g[i][j]);

        }

    while (q--) {

        scanf("%d%d", &rows, &cols);

        need = rows * cols / 2;

        size = 0;

        for (int i = rows; i <= n; ++i)

            for (int j = cols; j <= m; ++j)

                query[size++] = (i << 16) | j;

        printf("%d\n", search(0, mx+1, query, size)-1);

    }

}