Look at this picture of a valley in the mountains. A stream flows at the bottom of the valley. The valley land and the mountainsides are the watershed of that stream. The land is like a basin, or a bowl, that collects all the water from a rainstorm or snowstorm. Gravity then drags that water down into the stream, and gravity helps that stream flow out of the valley. This stream is a tributary to a larger river that eventually flows into the ocean.
You can make your own hand be a mini-watershed! All you need is your hand and a spray bottle with water in it. Just hold your hand up in a cup shape, like you see in the picture. Then, spray the palm of your hand. What happens at first? Where does the water go next? Does your hand have tributaries and a main stream? How is gravity working on the water in your hand?