In shutter speed priority mode, you manually set the shutter speed and the camera automatically chooses the correct aperture to properly expose the photo (ISO can be manually set or on auto).
In shutter speed priority mode, you manually set the shutter speed and the camera automatically chooses the correct aperture to properly expose the photo (ISO can be manually set or on auto). Aperture priority works the same way except you set the aperture and the camera chooses the shutter speed. In manual mode, you set all of the settings yourself.