What is PSM and is it mandatory to use it?

Hello, I’m new to the forum.
I’m designing a carrier board for my school research project with Pixhawk Cube Orange. I’m a computer programming student and I’m in love with drones. Making your own board for them is very exciting.
I have designed a board and I am having it manufactured. However, while designing, I saw the PSM- Power Selection Module.
When I wanted to get this module, I first saw that it was not available in my country or that the only place that sells it sells it very expensive (about 200 dollars each). When I want to get it from abroad, there are too many customs and shipping fees, too much for a student.
Yes, I looked at the PSM schematic from github, but since I am a computer student, it is not possible for me to understand.
I have 2 critical questions:
1- What exactly is the task of this PSM?
2- Can I run the cube and motors without PSM and fly my drone like a normal cube carrier board? Is there a way?

Thank you very much in advance.

@nihao the PSM module is required for doing the power supply logic and trigger appropriate actions during failsafe conditions like over voltage, under voltage, power cutoff on one of the lines. You can design a carrier board without PSM or something similar, but do realise that this will render your setup less reliable and safe.