Meta tags are an element of HTML (Hypertext Markup Language). They are placed in the head section of the web page's source code, describe the contents of that page and are used by search engines to index pages by subject.
Meta tags are also known as meta elements, description tags, meta description tags or metadata tags. The “meta” stands for “metadata,” which is the kind of data these tags provide – information about the HTML document. This information could be what keywords are relevant to the page, author of the document, page description, when it was last modified, what program was used to create the page, copyright information or any other details about the specific page. 
Meta tags are not displayed on the page itself, but only in the page’s code and are visible to search engines. (They can be viewed, with all the HTML code on a page, by selecting View/Source or View/Page Source from the browser menu.) 
Search engines use the information stored in meta tags to index a web page so that someone searching for the kind of information the page contains will be able to find it.There are several kinds of meta tags. The ‘title’ meta tag is self explanatory. The ‘description’ meta tag  includes a brief one, or two, sentence description of the page. The ‘keywords’ meta tag lists the words or phrases that best describe the contents of the page, but search engines today don't use the keywords meta tag in their ranking algorithms because they can be manipulated so easily by the web page writer.
When creating a web page be sure to include well written meta tags as they are useful for search engine optimisation and can help make the page rank higher in search results.