Animating in layers
If you're animating a character for the first time, you'll likely start by moving several bones and posing your character in the timeline until you're happy with the movements. That's quite similar to the "straight ahead" method in traditional 2D animation: this method is where you draw one frame after the other until you complete your animation, and it can enable you to achieve very expressive and fluid movements.
There's nothing "wrong" with that approach, but you may find yourself in trouble if you need to make changes to your scene after you have made all the poses. A good way to avoid such trouble and get a quicker feel of what your animation will look like before it's finished is working in "layers". If we keep the analogy of the traditional principles of animation, this method is more related to the "pose to pose" approach, where you define the key poses first, and then add the intermediate drawings.
Note
A quick intro to the 12 basic principles of animation can be...