Protocol in its most basic meaning is the set of rules defining accepted conduct between two or more individuals. The purpose of these rules is to ensure a smooth interaction by removing the chance of offense.
For a person or group these rules may specify such items as proper clothing, who speaks first and what title is used, who may sit or stand, and whose chair may be higher.
The term is used in communications and refers to the rules for communicating between two devices. The protocol defines what signaling is used and what each device is expected to do in any circumstance.
An example of a communications protocol is TCP/IP. This is actually two protocols Internet Protocol (IP) and Transmission Control Protocol (TCP). TCP/IP is the basis of the internet.