Meta Tags contain content that is in the header area of web pages. It is used for SEO (Search Engine Optimization) and contains keyword and description information for your site that can help people find your content when they use a search engine like Google.
There is a parameter at General > Configuration DEFAULT_META that you can edit to set the default meta tags for every screen on your site.
The Meta Tags screen is used to add meta tags for specific pages within your site.
Each meta tag entry contains a description field and a list of keywords. The list of keywords must be entered with commas between each keyword.
There is also a checkbox labeled 'Block search indexers?'. Check this box to add a meta tag that instructs spider robots to ignore the page. Spider robots are what search engines like google use to map websites. Note that nothing can prevent non-compliant search spiders from mapping your site so checking this box will only remove the listing from legitimate search sites that follow the guidelines regarding meta tags.
Note that meta tags are not as useful as they once were for getting your site ranked on search engine indexes.