Before explore the What function meta tags do ? to lets find out What are meta tags?. Meta tags are kind of HTML tags that placed into the header on a web page, between title and body tags. There are variety of meta tags that serves various jobs. Meta tags pops out with different context such as search engine optimization. Commonly used meta tags are meta descriptions and meta keywords. I think you might heard of these or might not.
Meta descriptions is a kind of tag that can feed to search engines with little bit of information about what specific page include. Most search engines feeding their databases from this little bit of information and its only about 150 to 190 characters. You have to summarize whole page content in to 160 – 190 characters. Sounds like little bit tough but it can do. Most Search Engine Optimizers doing this really great.
For example. meta description for this page is:

<meta name=”description” content=” Meta tags are set of meta information provider for search engines and for web browsers. Common Meta tags are meta description, meta keywords. More meta tags are there, read for more”>

The next most important and widely use meta tag is meta keyword. This meta tag  feed search engines with keywords. Keywords specifically to specific web page. Explaining further, take out the context of the page and divide them to keywords preserving the most important words for that page. Take out those preserved keywords and include them in Meta keywords section.
For example. meta keywords for this page is:

<meta name=”keywords” content=”meta tags, what are meta tags, meta tags information, meta tag, html meta tags, meta description, meta keywords, search engine meta keywords ”

There are many more meta tags. Here I’m going to mention some of those.
meta http-equiv tag – HTTP response headers, widely  using for  cache control, page refresh, character set identifying.
meta refresh tag – This tag giving information to browser to refresh the page and redirections.
meta robots tag – This tag feeding information to search engines whether to index or not to index specific page.
meta copyright tag – This tag gives information to search engines about copy rights.
meta author tag– This tag used to preserve the author information to the search engine.
If you need to view what kind of meta tags used for specific web page. Simply you can view them by right click and view source in any browser.
If you are interested in more technical details about metatags and how search engines need this follow these links as well.