Robots meta tag

Can someone please explain when it would be appropriate to use a robots Meta tag to tell search engines not to index a particular page. I just basically need an short explanation so that I can better understand. Any type of info would be greatly helpful and appreciated, thanks to all :) :)

Anything you don't want to appear in search results should be excluded. For example, you could have pages intended for specific people that you send that link to directly but you don't want them available for search.