I'm new to bitcoin mining. I'm trying to understand the role of the PSU.
The role of any
Power
Supply
Unit is exactly what the name says: It supplies the power a device needs to operate. In this case, it takes incoming 200-240VAC line voltage and drops it to a controlled 12 to 15vdc. Exact voltage out is set by the controller and is determined through a process called auto-tune where the miner controller goes through several cycles of setting a voltage & testing how well the chips perform. When auto-tune is done the actual voltage used is whatever gave best performance during testing.
As to how a PSU operates, that is an entirely different question and considering your followup question I highly suggest that you pickup a
Basic Electricity 101 book such as any of these found on Amazon . That will at least get you on a safe start to understanding how electrical devices operate...