Back in the metal type printing era, some glyph combinations were used repeatedly as they often occurred in some languages. They needed to be modified to have a combination that looks evenly spaced like the rest of the text (such as f and i). Punch-cutters created one type with the letters connected together instead of using two individual ones, and are called ligatures.

The same principle has been kept in digital typefaces and ligatures exist as independent glyphs. OpenType features allow us to switch from two separated glyphs to their ligature variant thanks to the ligature alternate features (if they exist in the selected typeface).