Combining a minor chord with a 9th (and a 7th) adds beautiful textures to the sound and depending on the position of the 9th, close or one octave apart from the minor 3rd, we can change the degree of 'tension' of the chord itself.
in the first example we add a F# on the first string (the 9th) to a E minor 7th chord. The minor 3rd is the open 3rd string (G) and so is (almost) an octave away from the 9th. Compare the sound of this chord with and without the 9th to hear the subtle difference in the sound.
In the other two examples we move a little bit down the neck: pay attentions to the roman numbers on top of the diagram that tell us on which frets we need to play these chords.
This time we add an F# on the 4th fret on the 4th string, just one half tone away from the open G on the 3rd string (we could consider this F# a 2nd). The closeness of the 2 notes gives the chord a particular tension.
In the third example we also add a D on the 2nd string to get a minor 7/9 chord.