The canonical version is the default, master, or authoritative version. Google has encouraged webmasters to indicate the canonical URL of a webpage, either by the rel=canonical link tag, or by consolidation on the server-side using a 301/ permanent redirect.

For example, say www.trwebco.com/example and trwebco.com/example are the same page. If they are both allowed to exist without consolidation, then it may be considered duplicate content by SEs.