It’s a set of rules for communicating data. The rules are defined for each process related to communication between any number of computers. Certain process can be handled by numerous protocols in the same time. Computer networks should follow these rules in order to transmit data successfully.
Example from the real life: the clock and the rules about how we measure time is a very good example of a protocol. All the countries of the world agree that there are 24 hours in a day, 60 minutes in an hour, and 60 seconds in a minute. It’s easy to see that if some countries chose their own protocols for dealing with time then it would be very difficult to get things done.