You can have a look at this. But a detailed explanation is given on this link.

Adding the Gist.

Any client (e.g. your Web browser or our CheckUpDown robot) goes through the following cycle when it communicates with the Web server:

Obtain an IP address from the IP name of the site (the site URL without the leading 'http://'). 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. Write an HTTP data stream through that socket. Receive an HTTP data stream back from the 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. This error occurs in the final step above when the client receives an HTTP status code that it recognises as '300'