ISOBUS is a protocol that determines how information is passed on the network and controls who speaks when: every controller listens to the CAN HI and CAN LO signals for packets and processes those packets that contain information that is relevant to its functions, ignores information it does not need, and transmits it's information (talks) when told to do so.  The CAN HI signal must match the CAN LOW signal as a redundancy means, if the two signals do not match it is considered an error; too many errors and the network gets mad and throws codes.

Maybe think of CANBUS as the phone line and ECUs as the fax machines (for those of you old enough to know fax machines).

That is a decent analogy.  A better analogy might be the old party line, where anyone that was on the line got to hear anything that was said and then used the information as they saw fit, with the difference being that there was a signal that told everyone when they could talk (each controller is allotted a time to transmit) so as not to jumble the signal with noise from multiple people talking at once.  A modern day reference would be that ISOBUS is like a larger conference call where everyone listens and only talks when told to do so.