It means what it says; the Apache software can't determine the hostname of your system. Edit your conf\httpd.conf file, look for the string "ServerName", and make sure there's an uncommented directive such as

ServerName localhost

or

ServerName www.foo.com

in the file. Correct it if there one there with wrong information, or add one if you don't already have one.

Also, make sure that your Windows system has DNS enabled. See the TCP/IP setup component of the Networking or Internet Options control panel.

After verifying that DNS is enabled and that you have a valid hostname in your ServerName directive, try to start the server again.