To add to Kyle's great and informative post about meta tags I thought I would share with you my theory about adding page descriptions via the "Page Settings" dialog.
All you need to make note of the dictionary's description of a meta tag, otherwise you will just be dding mindless keywords which, quite frankly, will look and sound like a big fat spam sandwich.
noun Computers .
a tag in HTML that is inserted at the top of a Web page chiefly to describe its content and provide keywords for use by search engines.
So, take heed, take time and describe your pages content, but briefly. It really helps if the content of your page is original in the first place, then naturally your meta tag will be more meaningful and visible in searches.
Content is king in SEO and meta tags are often used in search snippets too (aka search results text).
This is a great feature and is something originally asked for by your guardian Google Sites Top Contributors. So, let me know how you go with this new mega exciting new feature!