Usually, Google will use either your <meta description> or an extract of text from your page content as the snippet in its results pages. But sometimes, for whatever reason, it doesn't find either of these satisfactory, and so will use the description of your page in the Open Directory Project (DMoz) or the Yahoo! Directory. Using the 'noodp' and 'noydir' tags tells Google not to use DMoz and Yahoo! Directory to generate the snippet for that page.