It was a custom of Baroque composers to write the lowest melodic line - to be played by instruments such as the cello, the double bass, and the left hand on keyboard instruments - along with some additional figures that indicated to the keyboardist the chords they should use:
The section written for the instruments that play this bass line and those that realize it harmonically is called the basso continuo, or figured bass.
This custom was later abandoned, preferring that the composer write precisely what should be played; however, the figured bass is still used in the study of harmony today.