In figured bass, a horizontal line shows that the same chord continues. It is called a “continuation line”. Compare these two examples:
In A, a different chord would be needed for each of the three notes in the first bar (C major, D minor, C major in root position).
In B, the continuation line tells us that the C major chord on beat 1 continues throughout the bar, so the D is not actually a chord note at all (it’s an auxiliary note).
