I don't think I need to explain what a power supply does or how it works. It is the beating heart of your system, taking power from the socket on your wall or isobar and providing power to everything- How else is the computer going to work? Magic?
Here's mine- A used Corsair RX 850x, colored white.
Generally, it's bad practice to get a power supply that has been used. It could not only die on you, but take your whole computer out with it!
This power supply came from a trusted source, though. One of my bestest friends ever (who was also building a more impressive PC build than mine), also game me this unit along with the graphics card. So far it's going strong!