Slash-Chord. On the left side of the slash you have a chord, on the right side the lowest tone (bass). The Cm/Eb (say „C minor over Eb”) is a C minor chord with a low Eb played in the bass.
The Cm consists of the tones C (1), Eb (3) and G (5). In a slash chord the bass does not necessarily be part of the chord on the left. However, in this particular case the bass Eb is the minor 3rd (m3) of the Cm chord.
If you consider the bass Eb as the root, you can also see the chord as Eb6 chord with its 5th Bb not being played. On the other hand, if you would play the complete Eb6 chord, it would just equal a Cm7/Eb chord. Since in minor chords you can normally play or omit the minor 7th at will, the difference doesn't play a big role.
So you can say: Cm/Eb = Eb6
Example chord progressions: Fm7 Cm/EbBbm/DbEb Gm7 Bb/FCm/EbF For better readability the Bb/F is often written Gm7/F in this situation, although the G has been moved down to F and therefore there's normally no more F in the chord.