Water falling in a watershed drains to a common point.
Watersheds: A tributary at the arctic-boreal transition in the Agashashok River Watershed
A watershed is an area of land that drains all the streams and rainfall to a common outlet such as the outflow of a reservoir, mouth of a bay, or any point along a stream channel. Watersheds can be as small as a footprint or large enough to encompass all the land that drains water into rivers that drain into Chesapeake Bay, where it enters the Atlantic Ocean.
This picture gives a good representation of a watershed. It only shows a part of the Agashoshok watershed, but you can see how precipitation that falls on the near side of the ridges will move down-gradient to flow into the river in the picture.