It's described in the link in the entry: opting into the "site-wide canary" cookie gives you access to the most recent version of the code, including things that we haven't fully tested yet across all browser/operating system/configurations that our users use. Using Canary may fix that bug that's been bothering you for ages a few weeks before the fix is shipped to the site as a whole, or it may mean things are broken in entertaining and inventive ways!

If you want to test the homepage conversion, go to Beta Features, enable "Site-Wide Canary", and then visit www.dreamwidth.org while logged in to your account. You can compare it to the old version by visiting www.dreamwidth.org (while logged in to your account) without setting the canary cookie first. If you don't want to keep using the version that may break without warning, go back to Beta Features and disable "Site-Wide Canary" after.