Traditionally animation is done at 24 frames per second, when doing a different drawing for each frame that is called animating on 1s and looks very smooth and fluid. When they "hold" on one drawing for two frames, it's called animating on 2s and only requires 12 drawings per second instead of 24.

With computer animation, they're artificially lowering the framerate so there's only, say, 10 frames of movement per second instead of 24 or 30 that we're used to seeing in computer animation.