DRM stands for Digital Rights Management. You can think of it as an attempt to keep folks from copying the content, or you can think of it as an attempt to limit how you use the content you pay for. In essence, it's rather a hassle, and tends to keep new folks from coming into e-books in earnest, because they
don't want to hassle with it.
Or think of it as a way to protect the intellectual copyright of the authors, and to make sure they can earn their due royalties.