What ActivityPods effectively provides are automated mechanisms. They constantly check the contents of the Solid pod, and are notified whenever a change gets made.
Let’s say you’ve just made a post with your Fediverse app: a document representing a post is written in the Pod, then a dispatch mechanism acts as the user’s outbox and sends the activity out. Meanwhile, the corresponding inbox mechanism waits for replies.
What this could mean in practice is that editing a Fediverse post may be as simple as editing a corresponding file, while a mechanism pushes out an Update activity through your Outbox to make changes on the network.
I think I need an even more higher level explanation of Solid & Solidpods, but so far that sounds cool!
Would the data still live on your instance's server or on user devices? If it's the latter, how would it work if some people have really slow connections, or lose internet all together.