2D animation starts with clear plastic cells, on which an image is outlined in black lines. Each new cell presents those parts of the image which are intended to move, rather than the background or other motionless elements.
Once the drawings are complete, the cells are filled in with their proper colors using paint. That gives them a sense of depth and fullness which the line drawings lack.
The complete cells can then be laid on top of a static picture, containing the background and remainder of the scene.
The animator places the camera over one cell on top of the background picture, then shoots it. He then removes that cell, places the next cell in the sequence and shoots that.
Each cell represents only 1/24 of a second of screen time. When placed together in sequence, the cells gradually move the scene forward, creating the illusion of movement.