I am going to massively over simplify, but here goes. The really big idea from control theory is the idea of negative feedback. This basically boils down to measuring the output of your system and making the input to the system some function of the difference between the input signal and the output. The parent post refers to PID control. This refers to the three types of commonly used things to do with the difference (or error) signal.

P is for proportional - where you just multiply the error signal with a constant. What this does is to encourage the system to track the level of the input signal (but potentially with some lag)

I is for integral. This is where you integrate the difference signal over time. What this does is to reduce the lag between the input and output signal D is for derivative. This is where you feed back in the derivative of the error signal What this does is to damp down the swings in the system especially those that come from being too aggressive with the above two knobs.

Good controller design often comes down to picking the right weights for each of the three types of feedback functions you can input into the system. So in this example, it might be that you distribute your requests to servers based on how over or under loaded the servers are...