A canonical link element is an HTML element that helps webmasters prevent duplicate content issues in search engine optimization by specifying the “canonical” or “preferred” version of a web page.
Sometimes you have the same content in multiple pages. One is formated for printing, one is formated for mobikle and the same content in a downloadable PDF-format. This might confuse the search robot. By impolementing ‘canonical’ the robiot recognises which one is the original.