An HTTP status code is a three-digit number that tells the client how the server handled its request. Browsers and API clients read this number to decide whether the request succeeded, whether a redirect is needed, or whether an error occurred.
The first digit of the code defines its class. 1xx is an informational provisional response, 2xx is success, 3xx means further action (a redirect) is required, 4xx is a client-side error, and 5xx is a server-side error. So just the first digit already gives you a rough idea of the nature of the problem.
For example, 200 means OK, 301 means a permanent move, 404 means the resource is missing, and 500 means an internal server error. In practice, when you see a 4xx, checking the request (client) side first, and for a 5xx checking the server logs first, is the starting point of debugging.