It's the small box inside your computer that converts voltage from the wall into voltage your computer can use. It has a fan in it and it's usually at last half the noise you hear from your computer.
Dust tends to build up on the fan blades, these should periodically be vacuumed. Sometimes the fan will get very noisy; this isn't a terribly hard part to replace, just do it, but be aware there are capacitors within a power supply than hold enough charge even when unlpugged that they can kill you. So unlesss you now what you're doing, keep out of there.
When power suppplies fail they may do so all at once and the computer won't turn on or they may crap out when your system gets really busy or you may have masses of disk errors on drives that are otherwise ok. There's lots of fun symptoms of a weak or intermittant PSU.
The PSU that came with your computer is probably fine. But if you go adding stuff that needs cooling fans or extra drives save yourself some grief and double youw power supply right now.
There are two types of power supplies that I've seen. The first are painted, usually gray and are fairly light to hold. These are usually what comes with the computer originally and run from $20 - $50 depending on wattage.
Theen there are the ones that are shiney stainless steel with clear fans and friggin blue LEDs that light up when this beast is on and cost damn near or over a couple hundred bucks.
Yes these are rated at the same wattage as the other ones that are only $50. Yeah. Notice how heavy the expensive one is? Like chairs, binoculars and speakers, if it's heavy it's probably good.
See the thing is they both might be rated for, say, 800 watts but the cheap ones, if you select the right one, and tweak it a bit will actually hit 800 watts. On a good day. Then it'll probably blow up minutes later.
But the expensive ones rated at that same wattage mean that can run that load 24X7 for at least a decade.
The real test is to hook up an old floppy drive power connecter, but hook it up backwards. If after doing this and turning the power supply on the drive just no longer works, you have a not-so good PSU (and a dead drive now). But if there was a big flash and a lot of smoke and half the circuit card on the floppy drive has been vaoporized? Then you have a good PSU. And half a burnt floppy drive.
Ask me how I know.