SEO stands for Search Engine Optimization and is the process of tuning a web site to become "search engine friendly." This means structuring the site in such a way that a search engine crawler can easily make sense of the content on the site.
The value of the keywords tag is a comma seperated listed of words or phrases that are relivant to the information contained in the page. For example, the keywords for this page include SEO Overview, SEO Informaion, Silverlight SEO etc. The list should consist of words or phrases you would expect a person to search for when looking for the information on the page. The description tag's value is a brief summary of the content of the page. Some engine may use this as the abstract or blurb displayed with the search result for the page.
Here is a brief list of the SEO Basics:
Use meta tags - keywords & description
Use a title tag and a h1 tag for each page
Crawlers tend to treat content in b tags as important, usually good to bold keywords in content
Using the alt and title attributes for images and hyperlinks help SEO and accessibility
Make sure the keywords used in the meta tags are also in the page content
Using a robots.txt file helps instruct the crawler what to look at