Minify CSS (PageSpeed)
Minifying CSS files basically consists of removing the comments and spaces that were added to the CSS files for readability purposes, making the CSS files smaller.
Reducing the size of the CSS files translates into data savings for your website’s visitors.
With WP Rocket, you can easily address this opportunity by enabling Minify CSS files in the File Optimization tab.
You can also find more information in Google’s official article, Minify CSS.