...is the rain and melted snow/ice that falls onto our landscape and flows off of rooftops, streets, bare soils, and all other exposed surfaces. These surfaces can have pollutants on them (such as pet waste, dirt, litter, yard waste, chemicals, and auto fluids like oil and gas to name just a few) that will hitchhike with the stormwater as it travels, and then sadly the pollutants end up in our ponds, creeks, lakes, rivers and eventually Puget Sound. This is why all of us must to do our part to keep pollutants stored, used, and dispose of properly so they don't end up in our water resources and environment.
Check out this 5 minute video What is stormwater and why is it important? by King County that explains stormwater and its effect on the area.