The program (P) camera automatically determines the size of the shutter speed and aperture settings. Even so, we can change the combination of aperture or shutter speed.
The camera will try to balance between the shutter and the aperture, if we aim the lens to a bright area, the aperture number automatically enlarges while the shutter speed is maintained at a fairly rapid rate. Point the camera to a dark area and the aperture number will shrink to keep the shutter from blurry.