A "400 Bad Request" error is an HTTP status code that indicates the server cannot process the request due to a client error. This typically happens when the request is malformed or contains invalid syntax. For example, if a user submits a form with missing or incorrect information, the server may respond with this error to signal that it cannot understand the request. This error can occur for various reasons, such as incorrect URL formatting, unsupported characters, or exceeding request size limits. It serves as a reminder for users to check their input and ensure that it adheres to the expected format before resending the request to the server.