Essentially it's a mission mode where your character (and anyone you're grouped with) is dropped into a 24 player instanced area with only basic gear and a pistol.
You have to craft better gear, craft clothes to stay warm, and find survival resources (medicine, food, water) to stay alive while preparing to enter the final area to get a special item and extract. There are some neat bosses in the final parts.
There's a time limit as you're also dying from an infection, and the other players are taking the same resources as you.
You can also play it with PvP enabled for bigger rewards.
At the end, your character is rewarded a variety of loot based on how they did (even if they failed).
It takes maybe an hour tops for a run, though obviously can end much soon if you're doing well or fail.
Can you play in co-op with your friends? And do people try to create 8 player super squads in survival?