Leather tracing film is used to help transfer a pencil-sketched design onto leather. The original design, possibly sketched onto a piece of usual paper, can be traced onto the tracing film. When transferring the design to leather, the leather would be moistened, then the tracing film with the design laid over it.
Finally, a leather tooling stylus is used to trace the tracing film, which transfers an outline of the design pressed into it by the tooling stylus. This is quite a helpful tool for transferring sketches and artwork to leather.