ISO, shutter speed and aperture should be balanced to ensure that the photo is not either under or over exposed. The exposure triangle is used to show the correct balance needed and how each setting effects the others, for example there needs a low ISO if there is a low F stop.

Shutter speed refers to how long the lens is open for.

Aperture refers to how wide the lens opens.

ISO refers to how sensitive the sensor is to the light let in through the lens.