Digital Rights Management -- it's basically a way that publishers attempt to prevent wholesale copying of the work -- in other words: copy protection.
Trouble is, there are many approaches to DRM and they're not all inter-compatible. The other trouble is that DRM is mostly a nuisance to honest users, and less than a mild annoyance to those who actually
want to rip off the content.
If you have to enter a password, or if it's listed as "Secure PDF" or something similar, then it's DRMed.