DRM is an acroynm for Digital Rights Management, which refers to copy-protection technology that attempts to prevent protected digital material from being illegally duplicated.
One example of DRM is the encryption used on commercial video DVDs; another, the management system used by some digital book publishers that allows books to be viewed only by users logged into the accounts used to purchase those books.
DRM is fairly controversial for several reasons.
On one hand, media publishers rightfully want to prevent piracy of their work. Piracy amounts to theft, because it takes away the rights the publisher has to control how and where their works are made available. More importantly, it also impacts how much the publisher should be paid for a sale, by robbing them of those sales.
On the other hand, DRM punishes legitimate purchasers of protected content by preventing them from making simple backup copies, or converting the media for use on other digital devices also owned by the purchaser.
Ultimately, DRM tends to fail because it’s inevitably cracked or broken, allowing thieves to make illegal copies while still punishing the legal purchasers, who often need to restore to technically illegal measures to do simple things like make a backup.