Protocol in its most basic meaning is the set of rules to use when dealing with a particular entity. These rules cover such items as proper clothing, who speaks first and what title is used, who may sit or stand , and whos chair may be higher.
The term is used in computer communications and refers to the rules for communicating between two devices. The protocol defines what signalling 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.