If you're an all-grain or partial mash brewer, after the boil there will be a large amount of coagulated proteins and hops in the wort that you need to separate before transferring the wort to the fermenter or cooling cube.
"Whirlpooling" is a process that helps separate out this "trub", which can interfere with fermentation. Malt extract and kit brewers can use the same method to drop out hop residue.
Whirlpooling is as simple as stirring the wort in the boiler or saucepan in a circular motion to create a whirlpool. Then put the lid on the vessel and leave it for 10 or 15 minutes. The whirlpool will cause the trub and hops to be concentrated on the bottom and in the centre of the vessel, allowing the wort to be tipped, drained or siphoned off.
When creating the whirlpool do not stir so vigorously that aeration of the wort occurs, because admitting oxygen while the wort is hot (called hot-side aeration) can lead to the formation of compounds that cause unpleasant flavours and aromas in the finished beer.