Slash-Chord. On the left side of the slash you have a chord, on the right side you see the lowest tone (bass).
Here we have a Cm (C minor) chord with its minor 7th Bb in the bass. You might therfore write it Cm7/Bb. Since the function of a slash chords bass tone doesn't have to be mentioned on the left side of the slash, it's enough to write Cm/Bb. Sometimes the bass is even more the result of an independent bass movement and actually doesn't relate to the chord on the left in a special way (or the other way round: the bass stays on one note while some triads move independently).
This chord is often used in ballads, when a descending bassline but not much harmonical movement is desired, for example in the chord progression Cm Cm/BbAm7b5 Abmaj7.
You may have recognized that the
Am7b5 chord can bee seen as Cm/A and the Abmaj7 can bee seen as Cm/Ab. Thus this chord progression is actually just a Cm chord over a descending bass line.