Phono cartridges need some input capacitance to function properly, and the Muffsy Phono Preamp PP-4 lets you add capacitors to fill that need.
We're talking about small levels of capacitance, if any, from 0 and upwards to 200 pF. There is space for these capacitors, one for each channel, on the PCB, shown in yellow below:
As the capacitance of the cabling in the tone arm and your signal cables often will be enough (and in many cases, unfortunately, too much), you are advised to not install these two capacitors. How do you do that? Just leave the spots empty.
If you don't know if you need any extra input capacitance, chances are that you don't. Too much input capacitance will most often result in an audible higher level of treble.
There are a lot of resources on the Internet that discuss this topic in detail. That would be outside the scope of this project, so please consult the web for more information.