A simplified view of how browser rendering works
When a browser loads a web page, the first it has to do is read the HTML and then build a DOM tree from the content it reads. The next thing the browser does is parsing the CSS, which can be placed inline, embedded, or external and afterward the browser builds the CSSOM tree from what it sees. After the browser builds a DOM and CSSOM tree, it puts up a Render tree. And when the Render tree is built, the browser starts printing individual elements on the screen.
OK, but what is then render blocking?