On the Performance pages for your app, if you’re on a stack that counts requests, you’ll see legends in the Requests Per Minute graph saying “2xx”, “3xx” etc.
These refer to the status codes for the requests to your app. 2xx refers to successful requests (e.g. “200 OK”), 3xx refers to redirects (e.g. “301 Moved Permanently”), 4xx to client errors (e.g. “404 Not Found”) and 5xx for Server Errors.
You can read more in the Wikipedia article on HTTP Status Codes.