Depth of field (controlled by AV)
Aperture is one of the most important aspects of photography as it directly influences depth of field – that is, the amount of an image that is in focus. A large depth of field (achieved by using a small aperture) means that the foreground and background will be in focus.
A shallow depth of field (achieved by using a large aperture) would produce images where only the subject is in focus.
You would prioritise the shutter speed if you wanted to freeze fast moving action. Alternatively you can use slow shutter speed in a creative way to show motion tracing and a sense of movement.
Whilst the aperture and shutter affect the amount of light entering the camera, the ISO number affects the sensitivity of the camera to light. If you are shooting in low light conditions you can raise the ISO number, however this can reduce the quality of the image.