Meta Tag Overview
See some specific, DIY SEO tips here.
Title (Meta/HTML Title)
Usually 8-15 words describing the page. This text appears in the very upper-left corner of a browser window. Search engines typically use this as the link to the page in their search results.
Description (Meta Description)
Usually 12-22 words or so. Not displayed in a browser, but search engines show the description text under the title in the search results. Again, each page should have a unique description in phrase format, yet use the most important words on every page (or most pages).
Note on Keywords
Years ago, a Keywords meta tag was also recommended, however all major search engines ceased using them 4-5+ years ago and they are therefor unnecessary at this point.