Meta tags are HTML codes that are inserted into the header on a web page, after the title tag. The header is hidden code that only browsers and search engines can see. These tags take a variety of forms and serve a variety of purposes, but in the context of search engine optimization when people refer to meta tags, they are usually referring to the meta description tag and the meta keywords tag.
The meta description tag and the meta keywords tag were proposed so that webmasters would have a consistent method for providing meta document data to user agents, such as search engines. In addition to the well-known meta description and meta keywords tags, there are other useful meta tags, including the meta http-equiv tag, meta refresh tag, the meta robots tag, the meta copyright tag, and the meta author tag, among others. These tags are used to instruct user agents such as web browsers and search engine spiders on a variety of topics.