For animation to look realistic and believable, characters' muscles must squash and stretch as they move. Likewise, no living creature can move without planning to move first. Your animation demonstrates this by including moments of anticipation, however brief. For example, a pitcher must wind up before throwing a ball -- it doesn't just fly out of his hands. Staging involves planning not only the character's actions but also shooting in a way that communicates the basic events and emotion of an action with clarity and intention.
Straight-ahead and pose-to-pose are two major approaches to planning animation. Straight-ahead means beginning by drawing the first frame in a shot and then drawing subsequent frames without much structural planning. The resulting animation often looks spontaneous and surprising, but too much straight-ahead action leads to unfocused work. Pose-to-pose action involves drawing the first and last frames of a specific action and filling in the intermediate positions, or "in-betweens," along the way.
Pose-to-pose animation often serves the greater goals of a scene, but can look herky-jerky when overused. A combination of straight-ahead and pose-to-pose action balances planning and improvisation. Slow-in and slow-out refers to the fact that any physical motion in the real world begins and ends slowly. Using more drawings for the beginning and ending of an action than for the middle will reflect this.
When analyzed carefully, most physical actions follow motion arcs. For example, a ball thrown 30 feet falls gradually to the ground, creating an arc. Thinking of motion as occurring along an arc leads to more realistic animation. Secondary action refers to the way living beings often perform more than one action at once. For example, as a dog walks, its tail moves. Timing concerns observing actions in the real world and working to recreate them at the same pace, or changing the pace completely for emotional impact, such as showing an angry character running rather than walking.
The principle of exaggeration grew out of animation's roots in vaudeville and pantomime humor. Exaggerating your characters' physical actions -- from the way they walk to facial expressions -- drives home their internal states. Solid drawing demands an animator see her characters as three-dimensional creatures, and adds to realism. Avoiding contour drawing, the artist begins with construction lines and draws characters from the inside out. Though the "flat" or "modern" animation style of the 1950s subverted the principle of solid drawing, the animators working in that style had a deep understanding of solid drawing and consciously avoided it as a creative choice.
Appeal -- the most nebulous of the 12 principles -- is also one of the most important, as it refers to character design. Avoiding arbitrary choices and working with intention helps you design characters with emotional appeal to your audience. Every aspect of how your character looks and moves must convey meaning and evoke a reaction.