DRM stands for Digital Rights Management - a technical procedure which aims to make sure that the copyright holders retain control over their work by preventing unauthorized duplication of their work to ensure continued revenue streams. The Free Software Foundation (FSF) calls it Digital Restrictions Management. In recent times, DRM has caused extreme polarization even within the camps of Open Source and Free Software. With Linus Torvalds stating his displeasure over the move to formulate anti-DRM specific clauses in the upcoming GPLv3 spear headed by the FSF.
The big question is, when a lay person (such as your dad) approaches you and asks to explain what the cacophony related to DRM is all about, how do you explain in a way that he understands the basic idea of DRM without leaving him overwhelmed? If you are not sure how to address this question put forward to you, then you should read the example answers compiled by John highlighting the DRM-related problems, to help people understand what the big deal is with DRM.