The canonical meta tag defines a unique url to avoid dublicate content. Easy to understand with an example:
The content from
could possibly also be found through:
Searchengines see this as two different urls which show the same content thus penalizing one of them due to dublicate content. To avoid this issue you can solve it with the canonical link relation tag. In some cases this may be a tricky technical problem but it should be taken care of to concentrate link juice.
<link rel="canonical" href="http://www.yourshop.com/shoes/sports"/>
It should be correctly implemented within the <head> tag of the page and point to the version which should rank. The WordPress SEO plugin from yoast takes care of it by adding the canonical url to the header. However problematic queries like the search parameter from the example above require manual work.