The HTTP 400 status code indicates a "Bad Request." This means that the server cannot process the request sent by the client due to invalid syntax. Essentially, the server is unable to understand what the client is asking for, often due to errors in the request format or missing information. Common causes of an HTTP 400 error include malformed URLs, incorrect request headers, or invalid query parameters. When users encounter this error, they may need to check their input or the URL they are trying to access to ensure everything is correct before resending the request.