Overprint vs knockout

When one colored object sits on another, the press has two choices. Knockout is the default: the shape punches a hole in every ink below it, so it prints on bare paper and the color is exactly what you specified. Overprint skips the hole, so the ink prints on top of what is already there and the two mix: magenta over cyan gives you purple, not magenta.

Overprint is deliberate more often than people expect. Small black type overprints so a registration shift never leaves white halos around the letters. Trapping bridges the seam between two colors, sometimes with an overprinting stroke, more often with a spread or a choke sized by which ink is darker, or with a centerline trap, or by handing the whole problem to the RIP's trapping engine. Two-color and Risograph work overprints inks on purpose to invent a third color out of two plates. Varnishes, technical layers, and plenty of packaging builds overprint by design too. Knockout is the sane default for everything you have not deliberately decided about, which is not the same as everything else.

The famous disaster: white set to overprint. White ink usually does not exist in CMYK, so 'white overprinting' means printing nothing on top of the color below, and your text simply vanishes on press while looking perfect on screen. Rich black overprinting a photo is the other one, producing muddy patches. Turn on overprint preview in the layout app before exporting, and let preflight flag every stray overprint attribute.