Meta Tags are codes used in the web page source HTML. It describes what the page content is about and thus help the search engine identify relevance of your page content in a search. Just like tags used in a blog or article, meta tags help improve the visibility of your web page by describing its content, with the only difference that they are a part of the HTML and can only be seen by the search engine.
Different types of meta tags, used in SEO are:
Title tags
Description tags
Keyword tags
Robot tags