Squash & Stretch is a principle of animation used to apply emphasis and aid perception of animated object – eg, bouncy ball. A bouncy ball in animation in using this principle could lose its form when hitting the ground and become round again when it decelerates. It squashes and stretches. This shows the viewer the ball isn’t solid – it’s softer. This would be hard to convey without squash and stretch. In use while animating, this would mean elongating and narrowing an object because of a certain force, eg. Ball bouncing off the ground. Without squash and stretch, the ball could very well look the same as a rock.
Slight example of S&S from trailer below. 1:19 onwards