The meta description tag can be used by search engines tag content to generate snippets, if the search engine decides it to be better than page content. Social media sites also might use it for link preview if the page has no open graph description tag. A meta tag might be better if you want to control the summary of the page. In Joomla you can set a global meta description but this is not a good practice as search engines do not like duplicate title or meta description tags. The Global setting is in the Joomla Administrator at Administrator → System → Global Configuration → Site tab → Meta Data Settings → Site Meta Description
but it would be better not to use it.
TL:DR – The global meta description is an old global setting in Joomla. Search engines have moved on and so should you!