It’s a short description of a blog/page/post used mostly by search engines. This description is not displayed anywhere on the blog.
Here’s how Google uses it. Whenever someone googles a specific keyphrase Google makes a decision on which websites should be displayed and in what order. For each website Google displays a title and a short description. Google has two ways of putting this description together:
- If the meta description of the website contains the keyphrase used by the user then Google displays the meta description.
- If the meta description doesn’t contain the keyphrase then Google displays a fragment of the website’s content that does contain it.