Answer: A system of vessels in the brain that is responsible for washing out metabolic byproducts and cellular waste.
Outside of the nervous system, the body relies on the lymphatic system to pull waste material away for degradation. The brain uses a similar system that depends on glia (hence, glymphatic.)
First characterized in 2012, the glymphatic system is highly active when a person is sleeping. It uses the cerebrospinal fluid produced by the choroid plexus in the nervous system to wash away waste products.
It is believed that metabolic waste such as the amyloid-beta proteins found in patients with Alzheimer's disease is carried away and degraded by the glymphatic system. The implication is that better and more sleep may decrease the appearance of amyloid-beta plaques in the brain.