To fly forward the aircraft needs a device that will produce the thrust. This is the engine or the power plant. For general aviation, the engine produces
Power (P) (usually the thrust will be delivered through a propeller (prop)
or
Thrust (T)
For level flight the thrust must equal the drag, or power delivered must be balanced by the power required
Power P = T*V
In this course we look at air breathing engines. Hence , the performance of the engines will depend on the altitude.
You do not use full throttle for level flight. Even during take-off you will hold some in reserve