Google Sites uses the page description for various pieces of meta data, which includes the Open Graph Protocol (for Facebook Likes), Google-recommended schema.org microdata (for Google+ shares and +1's) and the standard description meta tag (search engines often use the meta description tag for search engine result pages snippets).
That's it, you're done.
You can get this feature now on stand-alone/consumer/personal Google Sites and Google Apps domains with the Rapid Release track enabled.
Do not try to fill the page description with meaningless key words, make it a true description/summary of the page's content. Search engines can easily detect spammy content so they can choose to ignore the page description. Search engines may even decide to display a snippet that differs from your page description if they feel your page provides better text elsewhere on the page that describes its overall aim.
You can find more advice, from Google, about page descriptions in the video below.
For more SEO advice see Google Sites SEO Guide.