ActivityPub is a social media protocol allowing federation. Which is an idea of linking two different social medias together. Think: liking and commenting on Facebook a post from Twitter. Of course neither Facebook nor Twitter are on the ActivityPub protocol. ActivityPub's social networks include things like: Mastodon, PixelFed and PeerTube. 
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More about ActivityPub you can find on <a href="https://en.wikipedia.org/wiki/ActivityPub">WIKIPEDIA</a>.