Meta tags tell the search engines a bit about your page.
Meta elements are the HTML or XHTML element used to provide structured metadata about a Web page. Multiple elements are often used on the same page: the element is the same, but its attributes are different. Meta elements can be used to specify page description, keywords and any other metadata not provided through the other head elements and attributes.The meta element has two uses: either to emulate the use of the HTTP response header, or to embed additional metadata within the HTML document.
There are three main things: Title tagDescription tagkeywords tagMost themes have an area where you can fill this out under each page or post. for example Thesis theme has this option built in. If your theme does not have this I would recommend installing WPSEO by Yoast as it is free and has many other SEO options. Spamming these areas of your site is not good.Here is a very complicated definition of meta elements from Wiki