What is a high impedance low level signal?
It is a weak signal.

High impedance means that the device or circuit that outputs the signal has a high output impedance. In simplified terms you could think of it as a high value resistor in series with the signal output (or the signal being sent over a long and thin wire). In real life there will also be some reactance and nonlinearity.

Low level in this context simply means that the signal has a low amplitude. This could be a result of the high impedance signal being fed into a low impedence input, but the way it's phrased, I think it's more likely to be an intrinsic property of the signal in question.