overprint
Overprinting is letting an ink lie over the ink already on the sheet, instead of wiping it off.
In more detail
A press lays one ink at a time and the sheet travels between them. Where a shape sits on a coloured ground, the machine has two ways of going about it. It can lay the shape's ink straight over the ground, and the two inks add up. Or it can leave the shape's outline blank in the ground — knock a hole in it, as the trade says — and print the shape into that gap.
Neither answer is right on its own. A hole keeps a colour clean, and it is what a pale shape on a dark ground needs; but the sheet has to come back to exactly the same place for every ink, and where it does not, a thin edge of bare paper shows around the shape. Laying the ink over hides that, at the price of the two colours mixing. Black type on a colour is the case everybody meets: it is laid over, because a hole would show a white halo around every letter.
What it does on a real document
The document says it, shape by shape. A declared graphics state sets `overprint` for both the fill and the stroke at once, or `stroke_overprint` and `fill_overprint` where a shape overprints on one side and not the other; a drawn shape then applies that graphics state by its name. Nothing of this shows on a screen: it is read by the machine that lays the ink down.
One more JSON key goes with it. `overprint_mode` says what a component of zero means while overprinting: `erase_zero` lays every component down as it is given, so a zero wipes that ink off the sheet, and `keep_zero` leaves the ink as it was found and lays down only the other components. That second answer is what lets black type with no cyan, magenta or yellow lie over a coloured ground rather than knock a hole in it.
Where it comes from, and why
The word is the workshop's own: the second ink was said to be printed over the first. It was settled at the film stage, before software, by whoever made the plates, and it stayed a conversation between the studio and the printing works long after the files went digital. Writing it into the document is what ends that conversation.