Stormwater runoff is water from precipitation that doesn't get evaporated or soaked into the ground. It is water that runs down to the nearest water source and pollutes the water with chemicals, dirt, and other items it has picked up on the way there.
The runoff can pick up many different chemicals that are dangerous for the bay to have in its waters. These chemicals include oil, fertilizer, sediment, and many more. The chemicals can cause algae blooms in the water. Algae blooms create dead zones (places without oxygen), causing wildlife to die. Runoff can even cause erosion if it moves fast enough, damaging the many wildlife creatures living in the bay.