Commons is where you can share course content or an entire course. You can share a page, a quiz, discussion etc.
You can keep it to yourself for reuse in new courses or you can share with colleagues – or with the rest of the world.

Therefor Commons is also where you can find learning resources from other users to import, customize and use in your own courses.

 

Commons Overview from Canvas LMS on Vimeo.

If you want to learn more about the Commons function, Canvas has a good introduction and guide. And you can learn even more about using Commons here.
Step-by-step quickguide

Download the Absalon guide here.