A wort chiller is a device that cools down wort (unfermented beer) after boiling. Cooling down wort rapidly is important because it helps to reduce the chances of bacteria from growing before you pitch the yeast. Yeast needs a certain temperature to live - too hot and you will kill the yeast, too cold and it may not ferment properly. Rapidly cooling wort is also important because it will help to discourage certain off-flavors in the finished brew.
There are mainly four methods of cooling wort rapidly. Immersion chillers are put into the wort and cold water is passed through to cool the wort. Counter-flow chillers are the opposite. They pass hot wort through cold water. Immersing the hot pot of wort in an ice bath is another way. You can also pour hot wort into cold water. The goal is to cool it as quickly as possible. Home brewers using partial-mash and all grain recipes typically find that wort chillers produce the best results.
My wort chiller passes hot wort through a copper coil submerged in a cooler filled with ice water and out through a ball valve on the side. The wort will drain out into a primary fermenting bucket where I can pitch the yeast and ferment the beer.
Step 1: Installing the ball valve on a cooler
Step 2: A word on safety
However, you should make sure to have all the materials you need on hand before beginning. You don't need to be driving a car to the hardware store for forgotten parts after following the golden rule.