Kief is a type of cannabis that has been extracted from the plant. When you smoke or vape weed, the kief is combusted and the smoke and vapor contain the active ingredients, such as THC and CBD. Kief can accumulate on the screen of your pipe or vaporizer, blocking its ability to vaporize your cannabis. To unclog your kief screen, use a pipe cleaner or q-tip dipped in cannabis-infused oil.