A grinder breaks down large pieces of plant matter into more readily smokeable chunks. Grinding plant matter makes it easier to load into a bowl, allows the material to burn more efficiently, and creates much better airflow. Grinders can have up to three chambers. he grinding teeth lock together, usually secured by a magnet, and the top piece of the grinder is twisted to crush the plant matter into a size small enough for them to fall through holes into the next chamber where it can then be loaded into a bowl or vaporizer, rolled into a joint, etc. If there is a third chamber, the bottom of the storage chamber would have a mesh screen which helps to collect kief. You can then unscrew the bottom chamber to access the kief once you have accumulated a decent amount.