Refer to figure.
The figure shows the changes that take place as the aircraft speed (TAS) increases. Assuming that propeller RPM and blade pitch angle remains constant, at a low speed the angle of attack will be large, giving a lot of thrust. Alpha will reduce with increasing speed and at about 4 degrees the blade will be operating at its best lift/drag ratio, i.e. the propeller is achieving its highest efficiency.

Higher speeds (maybe in a dive) will reduce the blade angle of attack to its zero lift value of about minus 4 degrees. At this point, the propeller will be producing zero thrust.

The thrust that is produced by the propeller depends on the angle of attack of the blade. The angle of attack of the blade depends in turn on the airspeed and induced airflow through the propeller. At low airspeeds the angle of attack will be large, near the stall, and thrust will be at its maximum. As forward speed increases, the angle of attack decreases and thrust reduces.

That means that the maximum thrust produced by a propeller driven aircraft, also occurs at zero forward airspeed and reduces with forward speed.

A fixed propeller can only have one optimum combination of forward airspeed and engine RPM and only one optimum propeller efficiency. At all other combinations it operates less efficiently. This effect can be offset to an extent by using a variable pitch propeller, coarsening the blade angle at higher speeds to maintain the optimum angle of attack.