Why N1 Low pressure compressor RPM and N2 High Pressure compressor RPM are monitored in jet engines ? Why specifically compressor RPM rather than turbine RPM ? What do N1 and N2 signify in terms of relation with any engine performance parameters?
Compressor and turbine are connected by the same shaft. You have LP compressor+turbine and HP compressor+turbine. For Rolls Royce engines you have N3 too because they use three rotors instead of two.
N1 or N2 show the percentage of rpm for the respective shaft. Each engine has a particular number of rpm corresponding to 100% N1 or N2 in order to make numbers easier to understand for pilots between different engines.