On this blog there are frequent mentions of render blocking javascript and render blocking stylesheets (or CSS). So what exactly is a render blocking stylesheet / script?
A render blocking resource (script or CSS file) is a file that has to be loaded before the webbrowser can start showing your webpage to your visitor.
All externally referenced CSS files are render blocking.
All externally referenced JavaScript files are render blocking unless you specify them either as async or deferred in the script tag.
If your scripts and stylesheets are very small, you can inline them directly into your pages. This will require them to be downloaded for each page viewed, but up to a certain page that is still going to be faster than referencing a separate file for each. Bandwidth is typically much less of an issue than connection latency.
If inlining is not possible / practical make sure to have as few script and CSS files as you possibly can. It is also possible to inline only the script and CSS needed to render the top of your page, commonly referred to as your above the fold content, and then load any additional scripts or styles later via JavaScript.