Join is a one day event about local multiplayer video games, which means “games that can be played by two or more people in the same physical location”.
From the website:
Why: Because the games that you can play together with other people are our favourite kind of games. Those games used to rule they world, then the internet arrived and we forgot about them. Now it seems like they’re coming back big time. To our knowledge, there hasn’t been an event completely dedicated to local multiplayer games yet, so we thought we should do something about it.