The minor 7th chord Cm7 („C minor seven”) is a C minor chord with an additional minor 7th Bb. You can get the minor 7th (7) by going up a minor 3rd (3 frets) from the 5th or by going down a whole-step (2 frets) from a root tone. A Cm7 consists of the tones C (1), Eb (m3), G (5) and Bb (7). The 5th can be omitted.
While major chords undergo a strong stilistic or functional (tendency to resolve) change by adding a 7 or maj7, you can feel free to play minor chords with or without the minor 7th in most popular musical styles.