I presume the Missions would be…Missions. (Not Quests.) 
Like these:
@Peterenp noted that maxing the Stronghold to 25 is likely one of them.
I presume they’d be related to building and farming materials, given the timing of their addition.
I’ll investigate more in Beta when it reopens, and it might be more obvious as we complete our Stronghold 21 upgrades.