The Description should be two to three sentences long and provide a fairly detailed description of what your site is about. Are you a florist in Denver? Then write something like:
Flowers and Such is a professional florist in Denver, CO with 25 years experience making birthdays a little brighter and anniversaries a little rosier. We have roses, lilies, orchids, tulips and every other flower under the sun.
The Keywords should be fairly targeted and also be the kind of words you use on your site. If your Keywords don't match your site content then they are practically useless. The imaginary florist we've written about above might want a list of Keywords like Flowers, Florist, Denver, Colorado, Birthdays, Anniversaries, Weddings, Apologise, Roses, Lilies, Orchids, Tulips. You may want to use around 20 or 30 words here that match your content and the types of searches people may do to find that content.
Note that Keywords are ultimately only helpful with search engines if they are words you actually use throughout your site (especially in Title areas). Otherwise Google (and other search engines) will just think you're trying to game the system by using Keywords that are irrelevant to your site and will just ignore them.