Slash-Chord. On the left side of the slash you have a chord, on the right side the lowest tone (bass). The Cm/G (say „C minor over G”) is a C minor chord with a low G played in the bass.
The C minor chord consists of the tones C (1), Eb (3) and G (5). The bass on the right of the slash doesn't necessarily be part of the chord on the left. In this the bass is the C major chords 5th.
Like many other slash chords, you may find the Cm/G where several triads are played over the same bass note („pedal bass”). Example chord progressions over G pedal: Cm/GGm Fm/GGm
or similar (Bb/G = Gm7) Cm/GBb/GAb/GBb/G
or for example Cm/GF/GFm/GCm
When you lower the root of Abmaj7 by 1 fret (a half-step), you also end up with a Cm/G chord. It will often be notated as Abmaj7/G in this situation (although there's no more Ab in the chord then), because written this way the player immediately recognizes that it's just the bass that is changing. Here is a chord progression starting off like that, but I'll stick with the notation as Cm/G: Abmaj7 Cm/GFm Fm/EbBb/DDbDbm/Eb