Stylistic sets (sometimes written as “style sets”) are groupings of alternate glyphs that allow the user to switch on (via OpenType) sets of related alternate glyphs one at a time.
According to the personal tastes of the type designer or foundry, each style set might contain multiple alternate glyphs—for instance a different “a,” “g,” and “z”—or the set might be limited to just one character.
If the type designer goes to the trouble of giving their stylistic sets human-readable names, they may be named according to the characters they modify (e.g. the “Arrows, triangles and circles” set in Alegreya) rather than the default set names (“ss01”, “ss02”, etc.).