Error 400, also known as "Bad Request," is an HTTP status code that indicates the server cannot process the request due to a client error. This error typically occurs when the request sent to the server is malformed or contains invalid syntax, making it impossible for the server to understand. Common causes of Error 400 include incorrect URL formatting, missing required parameters, or unsupported characters in the request. Users encountering this error should check their input for mistakes and ensure that the request adheres to the expected format for the specific web application or API they are trying to access.