Hinting

An outline is a mathematical shape and a screen is a grid of squares, and at sixteen pixels the stem of an h might fall across one and a half of them. Hinting tells the renderer what to preserve when it rounds — keep the stems the same width, hold the x-height on a pixel boundary, do not let this curve flatten. Done well it is the difference between crisp text and fuzzy text on a low-resolution display.

It matters far less than it used to. Modern operating systems largely ignore the fine-grained instructions in favour of their own rendering, and on a high-density display there are enough pixels that the question does not arise. It still shows up on Windows at small sizes and on older screens, which is enough of the world to be worth not stripping out.