The minor 9th chord - m9 respectively m7(9) - is a minor 7th chord with an additional major 9th (9). You can get the 9 on the guitar fretboard, when you go up a whole-step (2 frets) from from C (1) to D (9). However, it will be written as 9 and not as 2, because it is a chord extension (the number 2 only occurs in sus2 chords).
The Cm7(9) respectively C9 chord has the notes C (1), Eb (m3), G (5), Bb (7) and D (9). You can omit the 5th if you want.
Especially in jazz and latin music m9 chords are very common. Minor 9th chords can normally be played instead of a basic minor 7th chord, but you have to take care when it comes to the 3rd scale degree of a major scale, for example an Em7 in the key of C major. Here the next tone after E is an F (just a half-step away) and that is a b9. The b9 sounds just awful in a m7 chord and the 9 (F#) doesn't belong to the C major scale. Often a change of key is generated by adding a corresponding dominant 7th chord and voila! – the 9 works in that case, for example in the progression Em9 A13 Dm9 G13 Cmaj7.