Purpose

Higher CPU frequencies mean faster processing but also increased current, interference and maybe increased temperatures. Depending on the frequency range of your CPU, quality of your power supply and resolving capabilities of your entire system, you may benefit from changing the max CPU frequency that will be used for all your processing. By default, Euphony will run near max frequency all the time so the eventual balance is to be had if you lower the max CPU freq.

Using this option may have less effect if you are running Roon (instead of Stylus or some endpoint service) but it can be downright detrimental if you are using HQPlayer and do heavy upsampling.

Usage

The placeholder text for min/max CPU freq fields will show your frequency range: the min field will show the lowest possible frequency and the max field will show the maximum possible frequency - use these numbers as reference. 
Currently, the min CPU freq field is not utilized - you can change it but most of the time it will have no effect on frequency behavior as the frequency will always stay very close to the frequency defined in the max CPU frequency field (only if you enter same frequency into both fields you may force the system to have zero leeway in maintaining the working frequency). In most cases you can safely ignore min CPU freq field.
Most CPUs allow you to change frequency in steps of 100000 but some may have only a couple of frequencies that CPU can be set to. You will have to inform yourself about your CPU on the corresponding support pages of your CPU manufacturer. You will never get a response if your Apply was successful - you will have to check with Temp/CPU menu item to see what is the current frequency you are running at.

Risk level

If your enter wrong values the underlying system command will just fail, but if you enter a very low max CPU frequency your system may slow to a crawl or even become unresponsive!
Since the parameters are remembered, saved, and again used at startup - if you enter the wrong value and robe yourself of the opportunity to change it back to something better you may become stuck with that bad value and you will have to reinstall the system in order to get normal access again.