Kief is the fine, powdery collection of trichomes that falls off dried cannabis flower. If you have a three-chamber grinder, the bottom chamber with the fine screen collects kief as you grind. Commercially, kief is made by tumbling or sifting dried flower over fine screens.
Because trichomes hold most of the cannabinoids and terpenes, kief is stronger than the flower it came from. It is usually light tan to greenish, depending on how pure it is. The more plant material mixed in, the greener it looks.
People use kief by sprinkling a little on top of a bowl or into a joint, or by rolling a pre-roll in it. It is a common ingredient in infused pre-rolls. Kief can also be pressed into hash or pressed with heat to make rosin.
Kief can be potent, so start with a pinch rather than a big spoonful, especially if you have a low tolerance. Burning kief directly can be harsh, so many people mix it with flower.
Store kief in a small, airtight container away from heat and light. Over time it can lose aroma. Check the label and COA for potency if you buy it packaged.