I know. Especially on your first day. You're taking it better than most.

[He finally takes a sip of sake, and scratches Sumire behind the ears. The little cat purrs contentedly.]

Missions are... voluntarily doing tasks for the Malnosso in return for... rewards. [Strange note to that. It's more like he's likening these rewards to a treat you'd give a dog for compliance than any proper compensation for work well done.] Can be anything from retrieving the revived dead, combat, to willingly submitting yourself for experimentation. You just submit your name and they call you when they want you.

There's an entry on it in the back of your journal. It's explained in more detail there.