Bode Plots and Electrical Circuits - Zeroes and Poles
From your questions I understand that you want to understand how the feedback in buck converter works and chosen. 
For the analysis of the feedback network you should try understand three simple circuit and bode plots in general. (RC, LC and integrator)
The first is to get familiar and the LC and integrator are the basic parts of the buck converter.

So first lets look at the the RC circuit which is a first order system. 

obtained from https://www.robomart.com/blog/rc-filter-circuits-introduction

Now lets take a look at the transfer function (gain of the system):


Where s = jw

So analyzing the equation in frequency response : we see that there is pole (S is in the de denominator). 
So, the gain decreases as the frequency increases. 
More specifically, the gain decreases with a factor of 10 for every 10-fold in frequency. You can look into the definition of decibels that this corresponds to a decrease of 20dB/ decade. 

Now, lets look at the circuit more intuitively. 
So at low frequencies the impedance of the capacitor is very high and most of the voltage is across the capacitor (this is Vout).
As the frequency increases, the impedance of the capacitor decreases and thus the voltage across the capacitor decreases and Vout decreases. 
So, this corresponds to the decreasing magnitude of the frequency response.

now lets look at a second order system.

obtained from http://sim.okawa-denshi.jp/en/RLClowkeisan.html

So now we see in the denominator instead of S we see S^2.
If you look at the frequency response now the gain decreases with 40db/ decade after the corner frequency, because we have two poles instead of one.

for this case, it is a bit less intuitive: 
But lets try again , at low frequency most of the voltage will be across the capacitor as the inductor has low impedance. As frequency increases the capacitor impedance decreases but at the same time the inductor impedance increases, so this can somehow explain the second order dynamic.
But important here is : that in a LC filter the gain decreases by 40 dB after the corner frequency.

now lets look at the buck converter : 



Now look at the output we see the same  LC filter- stage. 

So if we want to design a feedback network, we can check the frequency response of all the stages. The whole point of looking this in the bode is that we can then easily add the gain of all the stages (filter, pwm and the switch). We can then add a feedback stage which alters the gain of the system in the frequency range of interest.
This is to much to cover here, but this should give you a basic idea.