Drainage basin is a geographic term about rivers. It is an area of land. All water that falls on the watershed flows into one river. It can flow directly into the river or go through tributaries (smaller rivers that flow into the bigger river) first.
The watershed of a river can be very large. For example, the watershed of the Mississippi River covers more than half of the United States. That means that over half of the United States is drained by the Mississippi and its tributaries.