Cm7 is a minor seventh chord made of 4 notes: C, E♭, G, B♭. It sounds soft and mellow, a minor chord with the edge taken off, and you hear it in soul, R&B, jazz and smooth pop, often as the ii chord. It is the i chord of C minor and turns up in 5 other keys.
The library above opens on Cm7 with 14 shapes up and down the neck, starting with hand-checked ones. Tap a shape to hear it, or play it one string at a time.