A pencil test is a step in the process of creating traditional (hand-drawn) animation in which the artist draws a sequence in pencil, which is then photographed and run as an animation.
This allows animators to see, assess and, if necessary, adjust the crucial elements of motion and timing prior to creating the more elaborate inked cells that are used for the final animation.
These are never meant for the public, any more than concept art or preliminary sketches; they are just intended to be tools for the animator. The images are often rough and characterized by smudges, false starts and scribbled notes.
The process is fascinating, though, and so is seeing the characters move and act, but in a rough state that makes it more obvious that these are moving drawings.
Pencil Test Deopt is a blog maintained by animator Jamaal Bradley, in which he gathers together pencil test videos from around the web and posts them along with descriptions of the piece and the animator(s) involved.
There are some pencil tests from classic Disney animations, as well as some recent additions from great Warner Brothers animators.
This is a tremendous resource for anyone interested in animation, and the process by which drawings “come alive”.