Render-blocking code above fold
Any code that will physically represent content above the bottom line of the screen it is rending on (called “above the fold”) may be preventing the content on the screen from showing up until such a time as the full volume of code is downloaded and executed.
In other words, if there is any code needed for the first view of the website, and that code is necessary for the screen to fill with content, then the time it takes for your user to see the page filled may take too long and this is judged as negative for UX and might affect your rank.
To minimize the effect of render-blocking code, make use of compression or otherwise Minify your code content, or make code load asynchonously to avoid linear processing queues. You may need a web developer to assist.
To test your pages for this potential issue, use the Google Pagespeed Insights tool.