In traditional hand-drawn animation, the "clean-up" phase (usually done by someone called a clean-up artist), is when the rough keys, breakdowns, and in-betweens are "cleaned up" into nice, smooth solid lines and forms. In a sense, the clean-up artist is tracing the animator's often rough drawings, but it's more than just tracing.
Even if you've never done traditional animation yourself, you've probably seen what rough pencil keys/breakdowns look like. Depending on the animator, they can be somewhat precise or thick, bold lines more concerned about movement than perfectly drawn body parts. Glen Keane, unarguably one of the best animators of all time, often has particularly "messy" breakdowns, but they are always very powerful, and clearly illustrate the pose, force, and movement in the body.
This is all a good clean-up artist needs to turn those rough drawings into the finished drawing we will see on the screen, but this is no easy task! The clean-up artist has to make sure the final line they are choosing is keeping the character on-model, is accentuating the proper facial expressions, is keeping things on nice organic arcs, etc. They work closely with the animator to ensure that the final product is what the animator envisioned, but it is truly a team effort, and the art of clean-up work is truly an art all on its own.
The best clean-up artist can take mediocre rough animation and make it look pretty darn nice, while a poor clean-up artist can ruin even the greatest animator's pencil roughs.
So, in traditional animation, the clean-up stage is incredibly important. But what about in CG?
At the vast majority of studios, animation is less compartmentalized for computer animation than it is for traditional animation. Whereas hand-drawn animation often would have an animator doing the keys and breakdowns, an in-betweener doing the in-betweens, and a clean-up artist polishing it all up, we usually wear all of these hats at once as a CG animator.
For us, the "clean-up" phase of the animation is pretty much exactly what the clean-up artist would be looking to accomplish in hand-drawn work. Once our animation is to the point where the timing is solid, the body mechanics are all working, the weight and balance is correct, the emotion is clear, the story points and character intentions are clear, and all of these things are nailed down, it's time to dive into our clean-up phase.
For me, the clean-up phase can actually be the most fun part, because it really can become a big puzzle to try to wrap your brain around. The biggest thing for me during this phase is to work on my arcs. In theory, my arcs should all be pretty close to working already (if I've done my planning and blocked my scene in correctly), but I almost always have some little arcs to fix or add, or find ways to make various arcs work better together (such as the arc the tip of a sword follows, and making sure that feels driven by AND interconnected to the arc the wrist is traveling along, which in turn should be related to the arc the elbow follows).
So, clean-up can be anything you do during the polishing phase. It's making sure the tip of the nose is on a subtle arc, or making sure all your face shapes are working together in a way to feel like one cohesive face rather than a ton of disconnected shapes. It's finding and fixing problems in your graph editor, or offsetting the weight just a tiny bit more to help your character feel that much heavier. It's fingers and toes and getting that nice whip action to look perfect on the character's tail.
Basically, in CG, people tend to refer to the "clean-up" phase as the time when you polish up all the little rough edges of your animation into a gem that truly shines. I can't stress enough that in my opinion, it's this attention to detail where you add that last 10-20% into your work that will really set it apart and help it stand out to recruiters and audiences.
Planning is a blast, because you get to brainstorm dozens of ideas and really work out the performance. Blocking is fun because you get to see those ideas finally come to life. But sometimes it's the clean-up or "polish" phase that can be the most rewarding, because all those little, tiny things you add can come together to take the animation to a whole new level, and even though an audience might not notice many of these little tweaks and overlaps and perfected arcs, they will FEEL them, and enjoy your work all that much more.
Hope that answers your question - Thanks for coming by the blog!