It’s the rules for how we interact. Like shake hands and introduce yourself. Many protocols literally have “handshake” stages.

I also like as an example the rule of not facing your back to the king/queen.

This is the original meaning of the word.

A protocol is just the rules to do something. Doesn’t matter of it’s a human language, a machine language, cleaning a room, driving a car, …

Protocols help computers know how to behave together. It’s like how you know that you make your voice go up to ask a question, so that I know I’m supposed to answer you.

you can replace the word ‘protocol’ with ‘set of rules’ in almost any context and it still makes sense.

Basic manners and politeness is a loose protocol of sorts. The following is a basic exchange between two people using please to request something, a response, followed by thank you and you’re welcome to wrap up the exchange. We do it that way because that’s the polite way of doing things, in other words it’s the expected protocol.

Can I please have X?

Yes you may.

Thank you!

You’re welcome!

For a five year old I’d keep it super simple and personally relatable and say it’s like a language that two people speak. Except it’s a computer language that websites speak (I often say websites instead of servers even when speaking to adults).

So like Facebook and Gmail can’t talk to eachother, but Gmail and Hotmail can. (You can probably come up with better examples.)

A protocol is a shared set of expectations that allow two or more things to communicate. It says “I will announce myself in this way” and “these are the responses you should expect to this request” and “here’s the list of all the things I know how to do”, and if we both follow the protocol, then we both understand what’s going to happen when we communicate.

A protocol is a set of rules that a group of people have agreed on to make it easier to talk to each other. I shut up when you talk and listen to what you say. I speak again when you have finished and then it’s your turn to listen. That’s a simple example of a protocol.

IP is more rules than that but it allows computers to talk to each other without having to exchange contract information first, like people do with phone numbers.

Let that sink in before you tackle Activity Pub.

C3PO is a protocol droid, and can “speak” many languages.

Computers use protocols like a language to understand each other when they connect

I guess I would explain it as an agreed upon way of talking to each other, say you decide you want to text, or call, or send a card. A protocol is like that. It’s not the data just the transmission method. A protocol controls the data stream, the hand shaking (when it is OK to start sending data, which port, where to send it). A data packet sent using a protocol is like a letter, with a sender, intended receiver, stamp (signature, key, CRC, etc.).

Start with non-computer protocols. What’s the protocol at a restaurant?

Patron: Open the door and enter. Wait 1.
If host currently busy Restaurant (Host): please wait. I will be right with you.
If host currently not present Restaurant (anyone): please wait, the host will be right with you.
If wait 1 times out Patron: excuse me, we’d like a table.
If host available and not busy Restaurant (Host): Hello and welcome. Wait 2.
if wait 2 times out Restaurant (Host): how many are in your party?
Patron: Hello. We would like a table state number of guests.

Or think about parliamentary procedure for bringing a motion to vote and voting on it.

These are protocols. They are all around us.

Computer networking protocols are so named because they are analogous. They define how to exchange information to accomplish a specific task. For IP, it’s a protocol for routing information packets across an unknown number of computers to a destination computer. For TCP, it’s a protocol for how a computer should send information packets and how a computer who is receiving those packets should acknowledge them.

Rules

Computers are really dumb, so when one computer wants to talk to another computer, they need an established set of rules about how each message will be sent and what to do with it when it gets received.

For IP, it’s mostly about how data is packaged and shipped. It has rules about how much data can be sent in a single message, some information about where the message is going and where it came from, whether or not they expect a reply, and then finally the block of data itself.

However, inside that block of data, there will be some more information about what that data is, so that the receiving computer knows what to do with it. This is where applications like activitypub do their work.