I'm receiving this randomly when I try to visit nvidia.com. I can either not reach the site at all with a 504 error, or I get this screen.
What does it mean, and what is the cause?
From the report by SSLLabs:
Chain issues Incomplete
This is a misconfiguration of the server: it is not sending the full certificate chain up to (but not including) the root certificate but instead only sends the leaf certificate. Specifically it is missing the intermediate certificate for RapidSSL TLS RSA CA G1. Without knowledge of the intermediate certificates the browser can not verify the certificate chain up to the locally trusted root CA - which results in the error you see.
While desktop browsers often successfully work around this configuration issues this is not successful in all cases.