Less a: "You need to know and get this game" and more a: "You should probably be aware of the existence of this game"
Second Life is a virtual 3D chat that allows you to converse with 'quite a lot' of people at the same time. It is notable, because it allows the avid user to create their own content, and sell it to other users for a profit. There used to be a somewhat equivalent on PS3 called Playstation Home, which was shelved some time last year or so, and has now two sudo follow-ups competing on PS4. How a chat can be a game? Well you can also create games inside of Second Life (most are garbage though), so it kind of works like a platform for games.
I've found these types of 'games' to be interesting, as they do something that is not frequently seen in games. Sharing creations (ranging from furniture to complete avatars) from all possible kinds of sources and synchronize them in near real time.