A one minute and 35 second search on google yields the following:
10.4.7 406 Not Acceptable
The resource identified by the request is only capable of generating response entities which have content characteristics not acceptable according to the accept headers sent in the request.
Unless it was a HEAD request, the response SHOULD include an entity containing a list of available entity characteristics and location(s) from which the user or user agent can choose the one most appropriate. The entity format is specified by the media type given in the Content-Type header field. Depending upon the format and the capabilities of the user agent, selection of the most appropriate choice MAY be performed automatically. However, this specification does not define any standard for such automatic selection.
Note: HTTP/1.1 servers are allowed to return responses which are
not acceptable according to the accept headers sent in the
request. In some cases, this may even be preferable to sending a
406 response. User agents are encouraged to inspect the headers of
an incoming response to determine if it is acceptable.
If the response could be unacceptable, a user agent SHOULD temporarily stop receipt of more data and query the user for a decision on further actions.
This error occurs very infrequently in Web browsers, because most browsers will accept any data returned from your Web server. If the client is not a Web browser, then anyone can only investigate the problem by looking at the Accept headers generated by the client system and the data stream returned by the Web server.