HTTPS Certificate Expiration will trigger a warning when a x509 certificate in the https chain is due to expire within one month (30 days).
HTTPS uses Secure Socket Layer (SSL) to encrypt traffic which means that data sent to or from your website is protected from 3rd parties intercepting and reading it. The certificate is issued by a trusted authority, like GeoTrust, which gives web browsers information about your site, including the domain name, your address, phone number, etc. This allows users to be confident that the site they visit is your site and not a data stealing 3rd party pretending to to be your site.
One of the best ways to get help if you are a MxToolbox paid member is to create a support ticket, and one of our staff will be happy to assist you.