Animation can be used for several different purposes such as movies, advertisements, video games, corporate presentations, medical concept demonstrations and much more. Not all of these require the same animation quality level. To illustrate a concept, simple and linear animation is often more than sufficient to convey the message. For a feature length animation movie, the animation needs to be convincing and carry some weight and elasticity.
You will notice when you watch different movies that the animation style will differ quite a bit from film to film. Some have a very fluid and smooth flow in their animation while others have a very bouncy, cartoony feel. People without animation experience may watch a movie and find the animation very bad, unconvincing or too slow. They might not know why exactly, but they know that something isn't right. Most of the time, it happens because the animators missed a few of the fundamental animation principles.
If you already are an experienced animator, you can skip this topic and continue along.
Here are the main principles that create the backbone of a good animation. You will no doubt encounter variations of these principles when reading articles online or reading different books.