There's no explanation for what the ControlNet feature is anywhere.
Viewing post in Retro Diffusion LITE Extension for Aseprite comments
It's a way of controlling how the image generates using some input images. You can read about it here: https://stable-diffusion-art.com/controlnet/ (This is about general controlnets and how they work)
Or you can see a video I did going over how they work in Retro Diffusion here: