When referencing “fractions” in typographic terms, we’re usually referring to the glyphs in any given typeface that are dedicated to the optimal rendering of the mathematical fractions themselves.
Fractions are much more legible when rendered correctly as symbols—e.g. “3⁄4”—rather than written out in regular numerals—e.g. “3/4”—so it’s desirable to enable this correct rendering via OpenType.
Even without OpenType, many fonts have Unicode points for the common fractions: ½, ¼, and ¾. Less-common fractions—⅓, ⅔, ⅛, ⅜, ⅜, ⅝, and ⅞—are not present in most fonts.