`this isn't the same kind of proxy as a caching squid proxy or such. what the apache proxy (mod proxy) will do is let one http server (i.e. your main example.com web server) invisibly relay pages from an internal web server. for example if you had a webserver running internally for some quiz say, and externally providing an address of
http://example.com/take/the/quiz then mod proxy would be able to recognise that particular path and invisibly communicate with the internal webserver, and just pass info back and forth, instead of serving the pages from it's own resource.
http://httpd.apache.org/docs-2.0/mod/mod_proxy.html