Short and dirty version: example.com has a certificate that is verified by a certificate authority (CA). Since there are lots of CA's, your browser cannot verify them all. However, it can verify a CA's certificate using another CA's certificate. A root CA is one whose certificate is included with the browser and is considered a trusted and secure source of identity information. Needless to say that compromising root CA's certificates in browsers is a big deal. On the other hand this particular instance in all likelyhood is simply a matter of human error, not a malicious attack.

Longer version: http://en.wikipedia.org/wiki/Public_key_infrastructure and http://en.wikipedia.org/wiki/Root_certificate