Meta tags provide information about the webpage in the HTML of the document. This information is called "metadata" and while it is not displayed on the page itself, it can be read by search engines and web crawlers.
For site's using the Web Theme, a meta description is added to each page. The meta description is a snippet of up to about 155 characters – a tag in HTML – which summarizes a page's content. Search engines show the meta description in search results mostly when the searched-for phrase is within the description, so optimizing the meta description is crucial for on-page SEO.