The C7(9), or mostly abbreviated just C9 is a dominant 7th chord („V7 chord”) with an additional major 9th D. The major 9th is the same tone as a major 2nd and therefore can be found 2 frets (a whole-step) above each root tone or 2 frets (a whole-step again) below the major 3rd on the guitar. But since it extends the chord it is written as a 9 in the chord symbol whereas the 2nd only appears in sus2 chords, where it replaces the 3rd.
A C7(9) respectively C9 chord has the notes C (1), E (3), G (5), Bb (7) and D (9). You can omit the 5th, especially if this makes it easier to play on the guitar.
In most cases a C9 chord can be played instead of a plain C7 chord. The voicing 1-3-7-9-5 (root on A string) of the seven-nine chord is one of the basic guitar chords of funk music.
Sometimes you have to be careful: if the C9 chord resolves to a F minor chord (m, m7, m9, etc.) you'll usually have to replace the natural tension 9 with an altered tension and play C7(b9) or C7(#9) instead.
If the m7 chord on the III. degree of a major scale has been changed to a V7 chord (the minor dominant), you'll also replace the 9 with b9 or #9, whatever chord it resolves to. In the following example you can see a V7 chord on the IIIrd scale degree (extended with a #9 here) and another one on the bVIIth degree, where the 9 is appropriate again: Abmaj7 C7(#9) Dbmaj7 Gb9