Survival is a new mission-like gamemode that you can play. You enter the gamemode, hop on a helicopter, helicopter crashes, you are left stranded without gear but for a green (as in scrub green) pistol. There is a PvP and a PvE version of this gamemode. Even PvE will be indirectly competitive because of limited resources.
this is a multiplayer gamemode that contains 24 players. Should you die, you're out. You still get some reward.
Your mission is to gear up, survive and get into the darkzone to extract some antivirals that might help fight the green poison.
you have to manage thirst, hunger, cold and infection. In order to manage that, you have to scavenge stuff that helps you, for example clothing to stave of cold. The infection can be held at bay by medicine but you will have to extract (yourself) eventually. That's where the upper time limit of 2 hours comes from.
rewards will include new (and old?) named gear
this is available today on PTS (PC only, console not ready yet). On the PTS you need to have the season pass to access survival. Specifically you need to have had it last thursday, when they did the import. That will get updated in the future.