A Root Certificate Authority is the highest level of digital certificate within the trust relationship of certificates. Web browsers, and other applications which use digital certificates, have a limited set of Root Certificates from organizations that have been recognized as Root Certificate Authorities. All certificates they create will include a link back to their Root Certificate so web browsers will understand that the certificate is valid and can be trusted.
Most applications that use digital certificates, such s web browsers, will have a list of the official Certificate Authorities so they are aware they are legitimate and trusted. Certificate Authorities who are not in this list will cause the application to display warnings that the Certificate Authority is unknown, and may also suggest there are security issues associated with unknown Certificate Authorities.