The critical path is the path to render a web page – what’s needed before that can happen. CSS Stylesheets block rendering. Until the browser has requested, received, downloaded and parsed your stylesheets, the page will remain blank. By reducing the amount of CSS the browser has to go through, and by inlining it on the page (removing the HTTP request), we can get the page to render much, much faster.
This tool extracts only the CSS needed for the above-the-fold content for the page you specify. You should minify this CSS, then inline it on your page (wrapped in <style> tags). It should be placed in the header – to replace the full CSS link(s).