Definition: Digital Electronic Communications Protocol
In the old days all electronics were analogue which means they used voltage levels to carry information. In those days the communication between 2 different systems or components consisted simply of a wire with a varying voltage, e.g. 1V would mean a pressure sensor is measuring an air pressure of 100kPA, 1.1V = 1.1kPA, etc.
With the mainstream move towards digital electronics, where information is represented as only a high or a low voltage (1's and 0's), a way was needed to send that sequence of 1's and 0's to the receiving side so that the receiving side can distinguish 2 subsequent 1's and not mistake it for a single long 1.
This is what a digital communications protocol is - it is a set of rules to enable different electronic components or systems to exchange digital information (1's and 0's).
Also see: Protocol Analyser