Defining Meta Tags is much easier than explaining how they are used, and by which engines. The reason is very few engines clearly lay out what they do and do not look at, and how much emphasis they put on any one factor. So, we’ll start with the easy part

Meta Tags are lines of HTML code embedded into web pages that are used by search engines to store information about your site. These "tags" contain keywords, descriptions, copyright information, site titles and more. They are among the numerous things that the search engines look for, when trying to evaluate a web site.