It uses a portable storage format to allow you to move user profiles, threads, and entire communities wholesale, between servers, as well as keeping them synchronized.
To use the town square metaphor, it's like meeting up downtown, but then inviting everyone to relocate to your place. (Except for the rude bigot and the annoying MLM promoter.)
It's a weird accident of information capitalism that we all signed up on platforms to talk with our friends and then those platforms gradually morphed into hyper-engaged outrage factories where you're getting sucked into political arguments with strangers.
What's striking to me about HN discussions of ActivityPub being the future is that the future is already here. The resistance already exists. Do you not know anyone who participates in a semi-private Telegram group or Discord instance?
Especially when you talk to younger folk they're acutely aware of how the behemoth, wide-open networks like Twitter and FB are dangerous places to be yourself. They don't participate heavily in these networks. They are already moving to loosely moderated, semi-closed communities on other platforms. These platforms have millions of users.
Now in light of that fact, Mastodon's role in the discussion seems to me to simply be "here is a similar solution which some techies are really into, it has federation which some of them think could be a killer feature, though it isn't yet."
If an individual is concerned about the effect of Twitter on society the best thing to do is quit using it, move to semi-open, semi-moderated platforms, and encourage everyone else to do the same.
I participate in Discord and Mastodon instances, I've cut back my FB usage and I wouldn't touch Twitter for a million bucks. This digital town square bullshit needs to do a lot better to earn my engagement. I'll just keep going to the local pub for that because at least I get to drink a beer when I'm there and people are usually more civil when they have to say things to my face.
I don't think we are screwed.. so long as we talk about and create better experiences people will see better possibilities and options. I just feel like we have a soup of platforms provided as services and open standards for communication platforms have been left in the lurch for a bit. When's RCS messaging rolling out? When was the last update to RSS to include commenting, tagging or broadcast? Where's the ability to reliably host video on your own server without uploading it somewhere else?
I actually think centralized social media causes a huge number of problems. Algorithmic discovery mechanisms specifically allow small numbers of highly motivated actors to spread fringe messages in a disproportionate way if they figure out how to game the system. That wouldn't be the case if the main way you found out about a new forum or news source was the direct endorsement of someone you trusted.
Many people say that they should consider Mastodon like email, but even then, there must be a forum where people need to meet in order to find like minded people and collect their email addresses. The moment such a forum develops, powerful people whose power is threatened by the meeting of such people will attack and destroy these meeting places. This will stop dissidents from getting their message out.
It was just a happy accident that town squares (where people were forced to come in order to buy and sell things) were owned by the government. Therefore a restriction on the government to stop it from restricting people to voice new ideas would mean that people would be exposed to new ideas. In this way, people were "forced" to consider new ideas or starve (If you did not go into the marketplace - which hosted the town square, you may starve as you could not get tomatoes or whatever. But if you did go, you will have to hear new ideas).
Today the town squares are Twitter, Facebook etc which are privately owned. If you cannot be on Twitter, Facebook etc you are screaming into the void. No one will hear your ideas. This will make the world less dynamic and less responsive to changing situations.
We are screwed.