HTML meta tags are special HTML tags which describe the nature and content of a webpage. Meta tags are not used by browser to display the text of the webpage, but are potentially used by search engines for indexing, ranking and displaying results in SERPs.
The most known meta tags in SEO are "description" and "keywords" meta tags. They were very important for good ranking in the SEO prehistory, but now they are no longer really taken into account for ranking.
The "description" tag can still be used by Google and other search engines for displaying the website description in SERPs. For more details on this topic see description meta tag.
Others meta tags relate to managing search engine spiders access, languages and content informations.
Matt Cutts speaks about the use of meta tags: