blend mode
A blend mode is the rule for the colour that comes out where one mark is laid over another.
In more detail
Lay a coloured square over a photograph and something has to be decided: does the square hide what is under it, or does it tint it? Both are useful, and neither can be guessed from the shapes themselves.
The plain rule is the one everybody expects — the mark on top wins. The others come from the darkroom and from the studio: one darkens as though two slides were held up to the light together, another lightens, another keeps only the difference.
What it does on a real document
The rule is stated in the settings that hold while a shape is drawn, so it applies to that shape and to nothing else.
A highlighter drawn over text uses the darkening rule, which is why the words stay readable through it instead of being buried.
Where it comes from, and why
The names come from photography, where a print was made lighter or darker by holding back or adding light, and were taken up by the first image editors on a screen.
Page formats adopted them once a page could hold see-through marks at all, so that a design made on a screen printed the way it looked.