The preprocessor significantly expands opportunities and provides more convenient syntax. The preprocessor accepts the data(your code stylus/sass/less) and gives (compiles to css)
A post - processor works with the css itself (post-processing - post-processing) for example framing vendorname prefixes.
In General, they need to combine for example
Please write stylus code
.container box-sizing border-box
it is compiled in
.container { box-sizing: border-box; }
next, we use the postprocessor(autoprefix) and obtain
.container { -webkit-box-sizing: border-box; -moz-box-sizing: border-box; box-sizing: border-box; }