Here's a start - proportional control is simply generating an output based on the difference between what you're measuring (or trying to control) and the setpoint.
Example: you want to control room temperature to 70 deg F - one input is the room temperature sensor. The other "input" is your goal of 70 deg F. Let's say the room is 65 degrees - the difference between 70 and 65 is 5 degrees - as the temperature rises the difference is less - so the output is less.
Some time ago I searched with Google and found a number of sites with lots of details. I don't have the urls but it didn't take long to find them. I probably searched on "PID algorithms" or something similar.