What are the lunar regolith?
The lunar regolith is the layer of loose soil that covers the surface of the Moon. The regolith consists of unconsolidated debris: dust, soil, fragments of the bedrock beneath and, as a result, is nonuniform in texture.
The word "regolith" marries two Greek words: rhegos, meaning blanket and lithos, meaning rock. If you can remember that regolith means "blanket of rock," it'll help you remember the regolith's specific characteristics, too.
Like a blanket, regolith covers almost the entire surface of the Moon, and is thickest on the lunar highlands (10 meters deep). On the mare, the regolith is closer to 5 meters deep in most places.
This famous photograph taken during Apollo 11 demonstrates the depth of the regolith. Walking through it might feel a bit like walking through very dusty snow!
Why is the regolith is so thick and pervasive? Since the Moon doesn't have an atmosphere of any kind, the surface of the regolith is directly exposed to the constant bombardment of meteors (not to mention solar wind). These impact events and harsh conditions break up the soil particles, melting and mixing the fragments of rock. Often, irregular clusters called agglutinates form due to this process.
Also thanks to this melting and mixing, the composition of the regolith is rich in the lunar rock type beneath it (basalt on the mare; pristine highland rocks on the highlands).