I tried looking online, but all I can find is what pitch ratio is, butI can't find anything related to the question above. Help will be appreciated.
Pitch ratio is important because it is a key parameter in propeller performance. As you have probably read already, it reflects the angle of attack of the blades, and thus their lift. Many propellers can very their pitch in flight to adapt to different requirements.
Look at the entire system: an engine produces torque, the amount of which depends on its rpm. The propeller converts said torque into thrust for the aircraft. The rpm at which the prop-engine system will settle is the one at which the torque from the engine matches the torque exerted by the airflow on the prop. This can be varied by altering the propeller pitch.
An additional use of pitch control is to "feather" the propeller, which is to place the blades in a least-resistance configuration when the engine fails, so the now useless propeller will generate the least possible drag.