Regular D is between 2-D (where images are flat, like in a cartoon) and 3-D (where you wear 3D glasses and images pop out at you). Most movies before 2008 were only available in Regular D.
The first commercials shown in both 3D and Regular D were aired during the 2008 superbowl.
Most TVs are only capable of showing 2-D and Regular D images.
"Hey dude - Avatar was sold out in 3-D. Is regular D okay?"