What does it mean "My digital certificate is revoked"?

Digital certificates can be revoked by the certificate authority that issued them. A digital certificate should be revoked when it has been compromised. Maybe we have lost the digital certificate, or maybe other people know its password, etc.

Once a digital certificate is revoked, future signatures made with it won't be valid. XolidoSign ask to the certificate authority that issued the digital certificate for the certificate revocation status to alert you if it has been revoked.