A piece of HTML code that provides a short description of a web page and is included in the code, but is not visible on the page itself. If a web page has a description tag, Google shows it if there is semantic similarity between the description tag and the content of the web page, and there is similarity between the user’s search query and the content of the description tag. If a page does not have a description tag, Google typically shows sentence fragments on the page that contain the search query.
Source: HSMAI CHDM Study Guide
