The meta description tag allows you to provide a summary of the content on the page in one paragraph. The tag can be used by search engines to display a description of the page when displaying it in search engine result pages (SERPs).
With the advent of contextual 'semantic' markup which allows a website to provide a context for the information, sometimes search engines will find more relevant information within the page itself or from other sources which link to that page (such as DMOZ) and use that in preference to your meta description. This does not mean that the meta description is not used, it simply means that if more relevant information is found to describe that page in search listings, a search engine may use that information rather than your specified description. In fact, Google recommends that every page on your site has it's own unique meta description. Google help on site titles & meta descriptions
When preparing meta descriptions for your pages, try to keep the total word count less than 155 characters, as there is a limit to the number of characters displayed in search results pages, so if you exceed this your text may be truncated. The meta description (if used) is one of the key factors that a person searching uses to decide whether your link is relevant to their search, so make sure that it makes sense to read, contains your keywords, and explains what is on the page. It is possible to dramatically improve your click through rate in search engines by making sure your meta descriptions are well optimised and relevant to the content on the page.
Google recommends the following to ensure that you gain the most from your search engine indexing:
Joomla! does indeed have a global field for meta description, which you can find under Global Configuration > Site > Global Site Meta Description. This will be used on any page within your site which does not have it's own meta description specified, so it can be a 'get out clause' if you don't want to create a meta description for each page, however search engines do not like duplicate title or meta description tags - using this approach you will inevitably have many pages with the same meta description.
Consider carefully if you wish to complete these fields - in most cases the meta description in global configuration can be left blank.