A device (usually a screen) located inside a marijuana grinder which catches and saves the keef, which is a high-thc substance that comes off the leaves when you grind up dank pot. This device is useful, because once you have the keef (which is very potent) separated, you can either smoke it straight or sprinkle it on top of regular nug in your bowl to enhance the quality of the nug and get higher. A grinder without a Keef Catcher can lead to wasted or lost Keef, which is just unfortunate.
-Hey, Calvin, I'm going to buy a new grinder. Wanna come?
-Nah. Sorry Rocko, I've got stuff to do. But make sure you buy one with a Keef Catcher this time, so we can save our keef and get extra ripped.