Canonical links are those links denoted by the HTML element 'rel=canonical'. This designation is typically used by webmasters as a way to inform Google that it should treat one page as the 'origination' page of the content, thus avoiding duplicate content issues if that content exists or had existed elsewhere.
According to Google, 'If your site has identical or vastly similar content that's accessible through multiple URLs, this format provides you with more control over the URL returned in search results. It also helps to make sure that properties such as link popularity are consolidated to your preferred version.'