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.