Basso continuo, sometimes just called "continuo", was played by an instrument providing chordal accompaniment such as a keyboard instrument or plucked string instrument such as the lute along with another bass instrument such as cello, violone, or bassoon. The keyboard instrument was normally a harpsichord but could also be an organ, such as a small portative instrument.
It was not usual to write out all the notes for the keyboard player. The composer normally just wrote the bass line which would be played by the left hand and doubled on the other bass instrument. The composer would indicate what the harmony should be (which chords should be played) by writing figures underneath the music.