Trub, pronounced “troob” is mostly composed of fatty acids and various proteins. Historically, it has been recommended to leave behind the break material (trub) from the kettle before putting your wort into the fermenter for fear of off-flavours. Check out this experiment done by Brulosophy for an in-depth look at the effects of trub in the fermenter.
If you use a plate chiller, hop material and trub can be a big problem, causing clogs. Make sure to leave as much of this material behind before running the wort through a plate chiller.
The Trub Trapper is a clever but simple design that helps to capture break material and hop pellets before draining your wort from the kettle. Check it out!