How webpack works. What are post plugins?

Question

webpack

Help please, the next moment in webpack is not clear. Heard advice on installing post-css, less. It was said that the scheme is simple. It is necessary to install post-css loader, post-css plugins, which we will transfer to this loader .... Why post-css loader is needed, I understand! It is necessary because webpack can only work with javascript and in order for it to understand post, we download instructions for working with post. But the next sentence is not clear to me. Why are these post plugins needed?