How can a signal itself have a high impedance?
How is it possible for a signal to have high impedance? How can the signal itself have resistance


You are technically correct that the signal itself doesn't have an impedance but the source does.

In the case of a piezoelectric sensor the transducer is pretty much a pair of capacitor plates attached to the piezo element which is an extremely good insulator and so has a very high resistance / impedance.


  Shouldnt the signals resistance / reactance be determined by which circuit is connected to?


You are on the right track. The signal level measured between the two devices will depend on the combination of the source impedance and the load impedance. The combination will form a potential divider. Too low a load impedance will reduce the signal level (and degrade the signal to noise ratio) and may cause distortion if the source is overloaded.