In figured bass, the number 9 is used to show a dissonance of a 9th above the bass. The rest of the chord will be the root position chord built from the bass note.
For example here, a bass note Bb figured a with a 9 means a chord of Bb major with an added C, which you can see in the tenor part.
The interval of a 9th above the bass is, of course, the same as a compound 2nd. Since it’s a dissonant interval, it normally needs to be prepared in the previous chord. “Prepared” means “used as a chord note in the previous chord of the same part”.
In this case, the added 9th is the A added to a chord of G major. The A is prepared as a chord note in the previous A minor chord, and then repeated as the 9th in the next chord.
9ths will often resolve downwards by step in the same way that 7ths do. However, sometimes this is just not possible, because the resolution note isn’t in the next chord. The best solution in this type of situation is to keep the same note in the same part, rather than changing it in any way.
The example above continues like this:
The A cannot resolve by step down to G in the next chord, because the next chord is D7 (first inversion) and therefore doesn’t contain a G. The A is continued instead, now as a chord note.