Download whatever one you want, extract the zip, and inside will be a set of images with stick-man/wire-frame looking figures. The same figures that the Pose model for ControlNet uses.
The problem is that normally, with the preprocessor enabled, it generates a black base for ControlNet, which will mean your pose is ignored.
Using the Pose model with a pose from a kit.
This is when turning on the 'Use Preprocessed ControlNet' comes into use.
This switch, when enabled, turns off the preprocessor. Meaning the ControlNet will just use the base image as its reference. You won't be able to preview the image with this on, because that part of ControlNet is turned off now. But it will follow the pose you have set as the base image
This will break most models if used with a regular image. But it is necessary to use when using a saved preprocessed image for any ControlNet model.
This is also a good way to make edits to the Canny and HED preprocessed images, to easily add or remove things with simple drawings.