Invented by
Osamu Tezuka in the 1960ies, "animation bank" is a name for time- and money-saving techniques in
animation, especially the reuse of standard scenes and poses, as well as the animation of only those parts of a scene that actually move, which can be just a mouth in some cases.
Now, while this may sound cheap and lead some people to believe that anime is the lazy man's animation, the important point is that it made many things possible that had previously been too expensive to implement, such as long-running TV series (in fact, Tezuka invented the animation bank while working on Astroboy).
Before Tezuka, each frame of animation was drawn separately, completely new. However, this meant that the animation either had to be very short, like the classic 30ies cartoons, or guaranteed to generate a lot of money - basically only Disney managed to make full-lenght animated movies.
So while it may seem a profane, unartistic method to save money at first glance, the animation bank was what made long, complex storylines and a diversity of themes in animation posible.