what is the difference between esModule option and CSS Modules for css-loader in webpack

I'm using css-loader v4 with webpack 4.44, I stumbled upon esModule option but I didn't understand how is this option different than modules option which when true enables CSS Modules ? and what is really exported with esModule: true anyway ?

import foo from "foo.css" // what does foo object in this code contains ?

foo.css for example:

.some-class{
    color: blue;
}
.some-other-class{
    color: red;
}