Chrisal,
A jet engine burns a lot less fuel at altitude than it does at sea level (this is because, as the altitude increases and the density reduces and so the mass flow through the engine reduces). In order to maintain steady state the fuel flow must reduce in proportion as the aircraft climbs.
In a simple (ie old) fuel control system the throttle controls fuel flow directly and so, as the aircraft climbs there is progressively too much fuel and the throttle must be reduced to prevent an increase in RPM. In a more modern engine there is provision in the fuel control system to compensate automatically for the reducing air density by making a consequent reduction in fuel flow and thus the RPM will be relatively stable.
A modern FADEC (Full Authority Digital Engine Control, ie a computer is in control) will of course adjust fuel flow to maintain RPM, EGT, EPR and a whole host of other parameters in the desired range.
Simplistic I know and I'm sure there will be many who will give a more detailed explanation but good enought for government work!
Early engines (de Haviland Ghost, RR Avon) had RPM gauges showing actual RPM of the rotor (there was only one); later, (RR Viper, Pegasus), %RPM gauges came to the fore; some display the N1, some the N2, in some there are possibilities to display both. A modern system (eg Airbus 320 with IAE V2500 engines) displays N1, N2, EPR, N1 vibration, N2 Vibration, EGT, Oil pressure and temerature, fuel flow and plenty more besides.
Rolls Royce publish a very good book on jet engines, it's called "the Jet engine", ISBN 0 902121 04 9. Don't know if it's still in print.
Hope this helps.
3 Point