Canonical meta tags are important to prevent duplicate content issues. Especially in ecommerce environments it happens that there are multiple versions of a product page due to various configurations of a product. Then it is important to define the page that you want Google to index. This is done with the canonical meta tag.
Roland Oberdorfer is a Managing Director and Co-Founder at SiteOlytics Inc. Prior to his current role Roland was the CTO of HP’s consumer direct organization and the General Manager for Web and eCommerce at NIVIDIA. You can find him on Google+ and on Facebook.