DRM = Digital Rights Management.
It's a 'protection' tool whereby the publisher imposes restrictions on the end user. The current iteration only allows you a limited amount of activations, and requires on-line activation of your game as well. This means you must be connected to the internet when you first fire up the game or it will ont run. If you run out of activations, you must contect tech support and request a new one, which they grant at their discretion. And any time the game needs to activate after that, you must go through the whole process again. New activations may be required when adjusting your BIOS, adding new hardware, or reinstalling your OS. Unfortunately, there is no information on exactly how much hardware change will trigger a new activation.
In addition, the DRM software may also conflict with other programs on your machine, requiring you to either shut down those programs, or even uninstall them altogether. As well, it sometimes is finicky about different types of optical drives (your CD/DVD drives) and will sometimes refuse to even load your game if you own a drive with which it has issues. There are also reports of it destroying drives due to the DMA step-down issue (this is a technical bit, google if you want more info).
Further, the software installs onto your computer without your knowledge (in most cases there is no indication that this will occur) or permission and hides itself from view. It can be difficult to remove and does not uninstall when you uninstall the game. There are reports of possible security issues with it, as well as reports of it conflicting with older versions of the security software, thus rendering some or all of your games unplayable.
But one of the biggest complaints against it is just the principle of it. This is purportedly to protect games from piracy, but it'spatently obvious it does no such thing. Why then, many people are asking, are legitimate customers forced to face these issues and problems when it does not do anything other than inconvenience the legitimate user?
These are some, but by no means all, of the issues with DRM.