In just a few lines, "Grand jury" gives us a cold, almost surreal picture of a committee meeting about crime. The people in the room are reduced to "raw heads," as if they were nothing but exposed, tender, slightly grotesque objects. They aren't speaking so much as "discharging one word / into a box." That could suggest a ballot box, a verdict dropped into a slot, or simply a container that swallows whatever it is given.
The speaker never says how they feel, but the tone is detached and quietly critical. Reducing human judgment to a single word, delivered into a box, makes the process seem mechanical and impersonal, even a little ominous. The word "discharging" adds to this, since it sounds bodily and bureaucratic at once. The mood is chilly and uneasy. The poem seems to wonder how much humanity is left in a system that handles people and their fates this way, though it leaves us to draw that conclusion ourselves.