Critical CSS are the styles necessary to load above-the-fold content on your pages. Having this allows your site to load quickly while requiring fewer resources.
This is great when users are clicking to new pages and will stay at the top of the page.
It runs into issues if you’re linking to content lower on the page (like headers, contents, or comments) as readers will see a flash of unstyled content, causing a “jump” as those styles are loaded shortly after.