Complex sentence: Key revocation can be added to the system by requiring a new certificate to be issued frequently (daily/hourly depending on level of security required), because the certificate is "public information" it does not need to be transmitted over a secret channel.
Split into simpler sentences: Key revocation can be added to the system by requiring a new certificate to be issued as frequently as the level of security requires. Because the certificate is "public information", it does not need to be transmitted over a secret channel.