Here's an engraving of a bit of Bach's St Matthew's passion. The bottom line marked "organo e continuo" is the figured bass. As you can see, it is one line of music (the lowest note) and the numbers, or figures, below the notes indicate which harmonies are to be used. The continuo player (usually a harpsichord) would then improvise all the other musical lines (usually three) which are to be played simultaneously with what is written.