A positive damping coefficient indicates that the cascade vibration in a specific mode is damped if left alone (stable). A negative damping coefficient on the other hand means that a once started vibration of the cascade will increase its amplitude (unstable) if not stopped otherwise (e.g. by mechanical damping). This condition is known as aerodynamic self-excitation. If the aerodynamic excitation is bigger than the mechanical damping the vibration amplitudes will increase until the airfoils break. This would be extremely dangerous in real turbomachines.