In the POH of the Cessna 172 it is advised not to go higher than 75% power in cruise flight (if I translated that correctly from German).
What does it exactly mean? What is 100%? Throttle cranked all the way up? So it is 75% of the throttle lever’s way?
This refers to the power output of the engine as percentage of maximum rated horsepower. In an old-school plane you won’t have a convenient read-out for this, but will have to use your RPM, speed, and altitude readouts in combination with the “cruise performance” chart in the POH.
The chart will show for various altitudes, at a few different power levels, what RPM you should target and what speed to expect.
I successfully ignored the chart so far, because these columns of numbers always make me dizzy. But I understand it’s time to give up ignorance and finally become a serious pilot. 
Thank you Vibstronium!