Brewers use the whirlpool process to separate hop particles and trub after their boil is complete. To prevent off flavors after fermentation, it's best to leave these particles behind before moving your beer to fermenter.
The goal is to move your wort in a circular direction until you develop what looks like a cyclone. This allows the hops and trub to settle in a cone in the center of your kettle, away from your drain port. And hopefully, those particles stay behind in your boil kettle!
For tips on how to use the whirlpool process, see here!