Hot break is the clumping of proteins, solids, and tannins that fall out of the wort during the boil and eventually collect at the bottom of the kettle. A steady boil is the key to achieving a good hot break, which will typically occur 5-15 minutes after the boil begins. When the foam on top of your boil finally dissipates, you know that many of the proteins have coagulated and that your hot break has occurred. A hot break is important because it aids in removing undesirable and potentially off flavor causing tannins and compounds from the boil. It also helps improve clarity and reduces the risk of chill haze down the line.
Below is a photo of the hot break in a 20 gallon brew kettle, approximately 15 minutes after the boil first began.