Trapping, overprint, and the black that is not black
Two adjacent colours will not line up perfectly, not on any press ever built. Trapping is deciding in advance which one loses, and rich black is the same decision made about the darkest thing on the page.
Registration is never perfect, so where two solid colours meet there will be a hairline of white paper on one side. Trapping is the deliberate overlap that hides it: one colour is spread slightly under the other, so a small misregistration eats into the overlap instead of showing paper.
Which colour spreads is not arbitrary. The lighter one always spreads under the darker, because the eye reads the darker colour's edge as the shape and will not notice the lighter one moving.
Overprint is the same idea, taken to its limit
Set an object to overprint and it does not knock a hole in what is behind it — it prints on top. For black text on a colour panel that is exactly right: the black prints over the panel, there is no hole to register against, and no misregistration is possible.
knockout panel prints, hole cut for the type, black fills the hole
→ 2 impressions must align to within ~0.1 mm
overprint panel prints solid, black prints on top of it
→ alignment cannot fail, because there is nothing to align
Warning
Overprint applied to white makes the object disappear. There is no white ink on a four-colour press — white is the paper — so an overprinting white object prints nothing at all. This has ruined more proofs than any other single setting, because on screen it looks correct right up to the moment it is output.
Rich black, and the number behind it
Plain black ink over a large area looks thin, so printers add other inks under it. The mix matters:
plain black C0 M0 Y0 K100 → correct for all type
rich black C60 M40 Y40 K100 → correct for large solids
registration C100 M100 Y100 K100 → NOT a colour. A press-alignment mark.
Registration black exists so the pressroom can check all four plates at once on the trim edge. Used as a colour it lays down 400% ink coverage, which will not dry, will set off onto the next sheet, and may tear the paper coming off the blanket.
The rule that prevents most of it
Plain black for type. Rich black for filled areas. Overprint black type on colour, knock out everything else. Never use registration black for anything you can see in the finished book.
Four sentences, and they are older than every piece of software involved in producing the file.