|The Effects of Gravity
Gravity is what pulls things down to the ground. If you jump up into the air, gravity will pull you back down to the surface. Jump out of an airplane and the same thing happens. Even it you were to be standing up and you suddenly blacked out for some reason, your body would collapse to the ground. Throw a ball up into the air and you know it's going to come back down. These are all things that we know and take for granted.
Because our drawings are lines on a flat piece of paper with height and width... two dimensional images, we use perspective to create the third dimension of depth. To keep our characters from looking like they're floating in space on the screen, we can incorporate a floor line to the environment and as long as our character doesn't move, they look like they're standing on the floor. As soon as the character has to move, we need to use the principle of gravity to make them look like they have weight and exist as we do.
The effects of gravity must relate to all aspects of the character's movements. If the character is walking, they must look like there is weight to their steps through the use of overlapping action in the hips, upper torso, head and arms. The addition of other elements to the character can help convey the illusion of gravity, such as clothing, hair, long ears (if it's an animal, like a dog). By having these overlap the action and drag as though they are being pulled down can add to the effect.