DRM stands for Digital Rights Management. It's a method whereby publishers encrypt their ebooks so that only the person that purchased them can read them, and limits on how many devices you can read that ebook. The goal is to prevent people from freely copying and distributing ebooks (i.e. the publisher receives no money).

Because of the encryption, you need special reader software that is able to authenticate you and the ebook and decrypt the ebook so that you can read it.