means A road surface made of layers of small, crushed, compacted stones, the engineered ancestor of the smooth roads we drive on today.

from Straight from the man himself: John Loudon McAdam, the Scottish road engineer whose method of building roads from carefully graded crushed stone was so successful that his surname became the surface. When tar was later added to bind the stones against dust and rain, the word grew a tail — 'tarmacadam,' soon clipped to 'tarmac.' So 'macadam' is one of those rare words that is simply a person, immortalized in gravel.