What's P-Factor and why does it occur?
For simplicity, assume a single-engine plane with a two-blade propeller. Just imagine an aircraft is perfectly level and moving forward through the air. The angle of attack on both blades of the propeller would be the same. Now imagine the plane pitches up a bit. One blade's angle of attack will increase and other will decrease.


(source: AOPA.org)  

Within reason, the greater the angle of attack, the greater the thrust generated. So one blade (or, to be more precise, a blade on one side of the aircraft) will generate more thrust than the other.