Hot and cold break are terms used by homebrewers to describe the floculation of proteins and other materials during the boil (the hot break) and cooling (the cold break). This material tends to settle to the bottom of your kettle or fermenter, where it becomes part of the "trub". Sometimes the terms "hot break" and "cold break" will be used to refer to the activity ("I had a great cold break when I pumped ice water through my wort chiller"), while at other times the brewer may be referring to the actual matter ("The cold break settled to the bottom of my carboy"); if you're worried that you may not be understood, you can always specify whether you're talking about the occurance or the stuff. Usually it is understood from context.