First, it is where elements are positioned in the authoring process. Second, it is where the action of the Director movie takes place. The score is timeline of the Director movie and is composed of frames and channels. Events happen through time in a Director movie depending on how cast members are positioned on the score. The movie starts on the left, and proceeds to the right through time one frame at a time.
Cast members can occupy one or more frames, thereby staying on the stage for that number of frames.
Time passes according to the frame rate of the movie. You set the number frames per second of the movie in the Control Panel. The movie's cast members are listed in the Score in channels, from top to bottom. You can have multiple cast members on the Stage at the same time by placing them in parallel channels in the Score. See the attached copies from the Director manual for additional information about the Score.