"The site's security certificate is not trusted!"
Google Chrome displays this warning if a website's security certificate isn't created by a recognized third-party organization.
When you connect to a website that uses SSL to transmit data, the server which hosts that website presents Google Chrome and other browsers with a certificate to verify its identity. This certificate contains information such as the address of the website, which is confirmed by a third party organization that your browser trusts. By checking that the address in the certificate matches the address of the website, it's possible to verify that you're communicating with the website named on the certificate, and not someone pretending to be that website.
Why you're seeing the alert
The third-party organization that issued your website destination's certificate is not one that your browser recognizes. Anyone can create a certificate claiming to be whatever website they choose. In order for Google Chrome to verify that you're connecting with the appropriate website, its certificate must be from a trusted organization.
If you're absolutely sure that your destination is trustworthy, you can choose to open the website by clicking Proceed anyway. In general, it's best to take precaution and click Back to safety, which returns you to the last webpage you visited.