An HTTP status code is a short number a website's server returns to describe what happened with a request. For example, the number can indicate success, a redirect, a problem with the request, or a problem on the server.
Think of it like a brief reply to a question: “Here is what happened when you asked for that page.” A browser's developer tools can show the code when a page fails to load.
The code can point toward the kind of issue, but it does not always explain the full cause. The app's logs or error details may be needed to understand what happened.