True Cinema is a term used nowadays as an indication that a HDTV set supports playback at 24 frames per second. With PAL video, it technically runs at a speed of 25 frames per second and NTSC video runs at 30 frames per second. However, the movie footage itself is usually shot at 24 frames per second, which means the video has been altered from its original speed to meet the requirements of PAL or NTSC televisions.
If your TV and the source of the video (i.e. a HD DVD player) support playing at the original framerate of 24 frames per second, then you can do that. Some users report a difference in quality while others notice no difference at all, or only do so on a title by title basis.