A canonical tag is a piece of metadata that allows you to communicate to search engines what web page was the original source of content found on a different page. A different page is designated by a different URL. Use a canonical tag when you need the user to see a specific URL (like when using query string parameters), but you do not want the user to see different content.
Note: With most modern CMS’s, it’s fairly difficult to mess this up because of the default settings, but if you’re dealing with an older site, be extra careful.
To learn more about Canonical Tags (how to properly and improperly implement, as well as how to find it on your site), signup for SEO Pro here