DRM is short for Digital Rights Management and what this means is basically copy protection.
Many Epub bookstores put DRM into the books they sell, and many times this copy protection locks the users into only using the particular bookstore's software for reading the book they have purchased.
You can find some stores that offers DRM-free books for purchase. There are quite a few online bookstores that does this actually and if you do a google search for "drm free epub" you will come across blogs etc. that have lists compiled with places where you can purchase DRM-free Epub books.
If you want, you can get pretty much any title that was released about 70 years ago since the copy protection can't last forever. When the copy protection time runs out, the title goes into what is called Public Domain. Many sites are dedicated to collecting public domain books that you can download. Inside ReadMe! if you go to Add Books in the menu and then tap on "Download books right now!", you will see a list of sites like this to get you started!
Read more here: https://readmei.zendesk.com/hc/en-us/articles/200988011-Can-I-read-books-from-iBooks-Kindle-etc-