I want to write something that sounds very simple because every post I heard of federation made it sound complicated which did stop me from trying Mastadon with the Twitter drama. But it's really simple that even a idiot like me can quickly figure it out that it's like talking to a next door neighbour only its a website like if Facebook, Twitter, Reddit etc could choose to directly interact with each other.
For a different examples imagine Nintendo, Xbox, Sony and PC playing a cross platform on a social game.