Let us start with the basics. DRM stands for Digital Rights Management. When applied to the context of PC video games, DRM is the method a publisher uses to verify that the game was purchased through a legitimate source and not pirated. Back in the early days of games, there was no DRM. I still have a few MS DOS games in a box somewhere that employ no method of verifying the game was actually purchased and not just illegally copied. As time went on, CD keys became the industry standard for DRM. Every game would come with a long string of numbers and letters that had to be entered before installing the game. Eventually, with the increasing popularity of the internet, those numbers were checked with an online server and if the same number was used too much, it was no longer valid.
But times are changing.