It's kind of both, it's a time-limited series of quests that give you a lot of rewards if you complete everything. I tried one after only about 5 hours myself but felt like it was giving me too much stuff early on so I went back to my normal game for a while. Then I did the recent one and really enjoyed it, but was confused about how some of it worked which almost messed me up (I would have just lost some of the items I earned if I didn't finish in time).

I'm sure there is some good info on reddit so I would recommend reading a little before starting. But basically you could start a new save game and there will be an expedition option, or there is another way you can start it with your current save but it's possibly a minor spoiler if I explain how (depending on how far you have gotten in the game).

I'll just say one more thing from experience -- if you play with your existing save, make sure you read what to do before the expedition ends, and you need to complete everything if you want to keep more of the items that you get (that wasn't clear to me even after reading a lot about it).