The domain name in the certificate received does not match the requested origin. The certificate may be a fully valid one, but was created for a domain other than the one requested.
What can happen?
Any modern web browser will reject the certificate so there are no security implications. The visitor will simply not be able to access the page and instead be greeted with an error, which is not very customer friendly.
The web server is likely misconfigured and uses a certificate from another domain. It is hard to give any general advice on this apart from advising to look at the web server setup.