Cm/Bb (C Minor over Bb)
Notes, from the bass up
Bb (bass)CEbG
A slash chord names two things at once. The part before the slash is the chord your right hand plays, and the part after it is the single bass note for your left hand. Read Cm/Bb as C Minor with Bb in the bass.
The bass note here is not part of the chord itself. Your left hand plays Bb while your right hand plays the plain C Minor shape above it.