Meta tags give search engines basic information about a page. The meta description tag may be displayed on search engine results pages in some cases (though usually they select a page area where the searched term shows up instead). The meta keywords tag may be compared with the keyword density on your page to verify the importance of keywords.

At Admin Panel -> Settings -> SEO -> "Meta tags", you have the basic meta tag configuration. The default content here uses conditionals to tell it to use the site description field as the meta description on most pages, but to use the category description or article description on category and article pages. It also uses the article tags field for meta keywords on the article details page, if you have tags switched on. You can modify all of this to your desire.

You can also configure meta tags at the category level. When editing a category, scroll down to the "Header Insertion" area... here you can supply custom meta tags for the particular category. Please note that these tags are used in place of the default ones, so you must add all desired tags and type out the full <meta name="description" content="x"><meta name="keywords" content="y"> tags. (If you do not enter anything into the header insertion field for the category, then the site default is used.)