Copyright is the exclusive legal right to publish, copy, and sell a work. It's usually granted to the creator of a particular work (which includes not only writing but also artistic works and performances, among other things) at the time of the work's creation, for a set term.
There is no requirement that a work be officially registered, marked with a copyright symbol, etc, in order to be protected by copyright in Canada. Just because something is freely available online does not mean it is not protected by copyright.
However, some works are in the public domain, for example because the term of copyright has expired; these works can be used by anyone for any purpose. Additionally, under Canadian law there are limited exceptions to copyright called fair dealing, which allow use of "short excerpts" for the purposes of research, private study, and education, among others.
Licensing is a means for a copyright holder to allow particular usage of a work. A license dictates the terms under which the work can be used, and what the person using it can or cannot do with it.
When you are publishing a new or adapted open access resource, you should choose an appropriate license for it. This licensing should take into account what you want to allow others to do with the work. You should also consider the status of the original work, if you've created an adaptation, or of any works by others (eg images) that you've included within your work – that licensing can limit your selection. Finally, keep in mind that in order to qualify as open, the licensing must allow for the 5 Rs. Creative Commons has a Choose a License tool to help you in deciding on an appropriate license.
Whichever license you select, make sure it is clear and visible to potential reusers.
Open access resources are made available under an open license, which allows for them to be accessed and reused with far fewer restrictions – in other words, they meet the 5 Rs. There are many types of open licenses, but Creative Commons licenses are the most common. There are six different "flavors" of CC licenses; all require that the original creator be attributed, but they vary in terms of whether modification, commercial reuse, and relicensing is allowed.