The browser does not trust whoever issued the certificate.
WHAT IT MEANS
The certificate was not issued by an authority in the browser's trust list, or the chain to a trusted root cannot be followed.
COMMON CAUSES
A self-signed certificate, which nothing vouches for A missing intermediate certificate, so the chain is broken A certificate from an authority that is not publicly trusted Security software on the visitor's computer intercepting traffic with its own certificate
FIXING IT ON YOUR SERVER
If self-signed: replace it. Run AutoSSL for a properly issued certificate.
If the intermediate is missing: reinstall the certificate including the issuer's bundle through cPanel > SSL/TLS > Manage SSL sites.
THE TELLTALE SIGN OF A MISSING INTERMEDIATE
It works in some browsers and on some devices but not others, because some already cached the intermediate from elsewhere. Mobile devices fail most often.
IF IT AFFECTS ONE VISITOR ONLY
Their antivirus or corporate network is inspecting traffic. Nothing at your end will change it.
CHECKING
An online SSL checker reports whether the chain is complete.
