Ligature

The classic case is f followed by i. In many faces the f’s hook runs straight into the i’s dot and the pair looks like a smudge, so the font carries a glyph in which the two are drawn as one shape with the dot absorbed. The reader is not supposed to notice; noticing means the ligature was designed badly.

These are standard ligatures and browsers turn them on by default in body text. Discretionary ligatures — the ornamental st and ct joins in some serif faces — are off unless you ask, which is right, because they are a decision rather than a repair.

Code fonts use the machinery for something else entirely: turning an equals-and-arrow pair into a single arrow, or a not-equals into a crossed equals. It is the same OpenType feature doing something the mechanism was never intended for, and whether you want it is a matter of taste rather than legibility — some people find it easier to scan, others find it dishonest about what is really in the file.