Simple Definition:

A protocol is a set of rules that govern how devices communicate with each other in a network. For example, the internet protocol (IP) is a protocol that specifies how data [information] is sent and received over the internet.

Very Simple Definition:

A protocol is a set of rules that devices use to talk to each other on the internet. For example, when you send an email or visit a website, your computer or phone is using the website’s protocol to communicate with the server.