What is a canonical url you ask?
There are two ways to access a web page:
They both get you to the same place. However, massive billion dollar companies with thousands of genius programmers (hint: Google) apparently cannot figure out this is the same website.
So what happens is that you get a link to www.kickbutt.im, and then a different link to kickbutt.im, and instead of getting credit for 2 backlinks to improve your search engine rankings, each "site" only gets credited for 1 backlink. One then cancels out the other as duplicate content and you just wasted a backlink.
What you need to do is tell Google this is the same website. You can do with canonical urls. This tells Google and links to one version of your url should be redirected to another version so only one version gets credit for the backlinks.
There is a useful tutorial on this page showing how to set this up.