@Marek I can so relate to your story, almost the same. The way I explained to my mother was: it is like playing a video game, but instead of sitting alone in front of the screen I play a game that comes from a book (my mother is a sucker for anything book related) with my friends. She accepted it, dough reluctantly
So about a decade later, when I was away on college, my mother came to visit with me for a few days. My friends came over for a night of D&D and persuaded my mother to join in as one of the NPS's

She got into the game so much it was hilarious, I was glad she had so much fun.
But that is not the funny part. At one point, her character killed an ogre by hitting him with a magic sword. She celebrated by calling my dad and sad: Gilbert, I waked an ogre on the head with a magic stick and now he is dead! I'm so happy!
Initial confusion was hilarious, my father calls my mother "Conan the magic stick" to this day