An error in an HTTP function typically corresponds to a standard HTTP error code returned by the API that you called. For example, if the API returns the status code 400, you get a "Bad Request" error. If the status code is 401, the error is "Unauthorized." You can see a full list of status codes, error names, and general descriptions in Wikipedia.
In addition to the status code, the data returned by the API will often include additional error information that can help you determine the cause of the error. For example, if you used an HTTP Get card to call OpenWeatherMap's API with an invalid key, you will receive an "Unauthorized" error. You will also read the message "Invalid API key" along with a link for more information.