以下代码请逐句详细解释以下，谢谢！

function getCookie(cookie_name) {
                var allCookies = document.cookie;
                var cookie_pos = allCookies.indexOf(cookie_name);   //如果找到了索引，就代表cookie存在
                if (cookie_pos != -1) {
                    cookie_pos += cookie_name.length + 1;
                    var cookie_end = allCookies.indexOf(";", cookie_pos);
                    if (cookie_end == -1) {
                        cookie_end = allCookies.length;
                    }
                    return unescape(allCookies.substring(cookie_pos, cookie_end));
                }
                return null;
            }