Figured bass, or thoroughbass, is a kind of musical notation in which numerals and symbols (often accidentals) indicate intervals, chords, and non-chord tones that a musician playing piano, harpsichord, organ, lute (or other instruments capable of playing chords) play in relation to the bass note that these numbers and symbols appear above or below. Figured bass is closely associated with basso continuo, a historically improvised accompaniment used in almost all genres of music in the Baroque period of Classical music (1600–1750), though rarely in modern music.

https://en.wikipedia.org/wiki/Figured_bass