As anyone who has tried to pursue even a little bit of academic research can attest, publishers charge an arm-and-a-leg to access studies if you are not part of an institution that subscribes to their journals. And the authors of those studies don’t even get any of that money!
There are millions of research papers available that are freely available but for some reason you may find them behind a paywall. In this instance, a paywall is a website that requires some form of payment or registration to access materials. The Unpaywall extension will find the paper if it’s available for free.