Critical CSS is a performance feature that was added in Avada 7.4. You can enable this in the Options > Performance tab, or when going through the Performance Wizard. Once it has been enabled, you can access the Critical CSS page from the WordPress sidebar at Avada > Critical CSS, or at Maintenance > Critical CSS from the Avada Dashboard.
Both take you to the Critical CSS page, where you can generate Critical CSS for both pages and posts. Critical CSS is the CSS necessary to style the above the fold content. When generated for a page, this will be loaded into the page head, with the rest of the styles moved to the footer, which should mean the above the fold content is quicker to be styled and there is less render blocking time.