In a traditional hand-drawn animation, normally, a collection of frames played at 24 frames per second, so a minute's animation is 1,440 frames. So, extrapolating 20 minutes of animation adds up to 28,800 frames of animation.

Of course, in most cases, this is way too much work for a single artist. So, a lead artist will draw the signature frames. These might be the superhero's main poses or the sweeping landscapes' endpoints. These important frames called keyframes. Once the keyframes are laid out, the junior artists will fill in the frames between these keyframes. This process is known asin-betweening, or simply tweening.

Keyframing is the single most important concept to learn with regard to animation. I have never in my life seen an animation tool that does not use it. This is how most of the animation is done in the world, whether hand-drawn or electronically rendered.

Nowadays, you are the lead artist setting keyframes. The computer will do the tweening. There are a lot of options to ensure that these frames are correct. For now, we will keep this simple.