C minor 6th chord. The Cm6 chord is a minor chord with an additional major 6th. To get the 6th (A) go up a whole-step (2 frets) from the 5th or a minor 3rd (3 frets) down from a root tone. You can also take a Cm7 chord, find its minor 7th Bb and move it just 1 fret (1 half-step) downwards to the major 6th A.
The Cm6 chord consists of the tones C (1), Eb (m3), G (5) and A (6). The 5th (G) can be omitted.
Common chord progressions with the Cm6 chord:
The II-V-I progression Cm7 F7 Bbmaj7 can be modified to Cm7 Cm6 Bbmaj7. The Cm6 chord contains the same tones as a F9 = F7(9) chord without the root F. Thus the Cm6 chord has a similar tendency to resolve into the Bbmaj7 chord as F7 respectively F9. The intensified minor character (sad, sentimental,…) goes along with the tranquilized movement of the bass.
Another chord progression with the same intention: instead of Cmaj7 F7 Gmaj you are playing Cmaj7 Cm6 Gmaj7.
A different application are the so called line-clichés like Cm Cm(maj7) Cm7 Cm6, where just one single tone is moving up or down chromatically.