有一点不太理解

#include <iostream>
#include <deque>
using namespace std;
const int N = 5e5 + 5;
int n, m, ans = -1e9;
int sum[N];
deque<int> dq;

int main() {
	cin >> n >> m;

	for (int i = 1, x; i <= n; i++) {

		cin >> x;
		sum[i] = sum[i - 1] + x;
	}

	dq.push_back(0);

	for (int i = 1; i <= n; i++) {

		while (!dq.empty() && i - dq.front() > m)<--这里
			dq.pop_front();
		ans = max(ans, sum[i] - sum[dq.front()]);

		while (!dq.empty() && sum[dq.back()] >= sum[i])
			dq.pop_back();
		dq.push_back(i);
	}

	cout << ans;
	return 0;
}