The torque bandwidth is typically referring to the maximum frequency of motion at which the actuator can provide that torque. So your actuator can provide a peak torque of 100 Nm, as in it can hold up a weight of 100 N held at a torque arm of 1 m. If you want to swing that weight back and forth you could do it at up to 4 Hz, but no faster without damaging or overheating the actuator. However, if you reduce the weight to 2 Nm then you can swing it at 20 Hz.
Keep in mind that this is an oversimplification since if you were swinging a weight back and force the torque will not be constant the entire time. Still, it provides you with an idea of how fast the actuator can respond with different levels of applied torque (the torque is a resistance against the actuator, or rather an "impedance").
Note the "zero impedance" bandwidth is the same thing except with no payload weight being held at the end of the actuator torque arm -- the weight of the arm itself is the payload.