400 Bad Request

The definition of this error (officially) is "Invalid request".

Short version: The web server thinks that the data stream sent by the client (e.g. your Web browser) was "malformed" i.e. did not respect the HTTP protocol completely. So the Web server was unable to understand the request and process it.

Long version: Any client (e.g. your Web browser) goes through the following cycle:

Translate the name of your site (your site URL without the leading 'http://') to an IP address. This lookup (conversion of IP name to IP address) is provided by domain name servers (DNSs).
Open an IP socket connection to that IP address (i.e. contact that web server and yell "hello" at it, and get a "what do you want?" back from it).
Write an HTTP data stream through that socket (i.e. give it your request)
Receive an HTTP data stream back from your Web server in response. This data stream contains status codes whose values are determined by the HTTP protocol. Parse this data stream for status codes and other useful information.
If the response is valid, display the web page.

This error occurs in step 4 above when the client receives an HTTP status code it recognises as '400'.