Survival Mode is an endless mode in Tribes of Midgard where players can hold out against endless hordes and explore the world of Tribes of Midgard and everything it has to offer.
The main difference from that of the saga mode is that player's progress only ends if they fail to protect the tree, and the duration of the game will also last as long as players are able to protect this village tree.
The endless winter season will eventually cycle back into the summer season as well in a summer and winter season cycle.
Along with that, In survival mode, players will not make any progress in the season rewards pass or will be able to unlock any achievement or class that lists a requirement of saga mode.
What this means is if you want to complete specific achievements or unlock a class, you will need to do so with the main saga mode instead.
Overall, survival mode should be the mode to play when you want to sit back relax with some friends after completing all of season one's saga content and have unlocked most of the achievements.
Survival mode can also be a way for you to improve your skills at the game without fear of messing up and failing the entire run, like in saga mode.
That said, if you want to play survival mode, you'll need to play at least one sage mode, and the game mode will unlock for you.
Check out some more Tribes of Midgard guides at Pro Game Guides!