This issue is triggered if browser caching is not specified in the response header. Enabling browser caching

for JavaScript and CSS files allows browsers to store and reuse these resources without having to download them again when

requesting your page. That way the browser will download less data, which will decrease your page load time. And the less

time it takes to load your page, the happier your visitors are. For more information, please see this Google article

https://clicktime.symantec.com/3Wg2xrqzkUF5QQ9G6rAKDD47Vc?u=https%3A%2F%2Fdevelopers.google.com%2Fweb%2Ffundamentals%2Fperformance%2Foptimizing-content-efficiency. How to fix: If JavaScript and CSS

files are hosted on your website, enable browser caching for them. If JavaScript and CSS files are hosted on a website that you

don’t own, contact the website owner and ask them to enable browser caching for them.

 

However, some time you will find that after your are aware of server and client side cache but http requests as above example referring to belongs to some pixels or analytics tools which can not be cached on client side.