Tina, your shutter speed, ISO and aperture all work together to obtain proper exposure. The aperture is the size of the opening in the lens to allow light through to the sensor. The shutter speed is how long that opening stays open. The ISO is the setting for how sensitive your shutter is to the light coming through to it. So some examples of how the work together (these examples assume nothing changed like the sun going behind a cloud or it being close to sunset where the light changes constantly).
Say to get a good exposure you are at an aperture of f8 and a shutter speed of 1/500 second at ISO 100. If you change one of these, say ISO to 200, then the others need to change to maintain that good exposure. In this case the ISO doubled, so the sensor will be twice as sensitive to light. You will either have to leave the aperture open half as long (half the shutter speed to 1/1000 second) at f8 or leave the shutter speed the same and close the aperture by half it's size (a full stop to f11).
So, in a case where you take a picture and it is underexposed, you have three options...(1) increase ISO, (2) open up to a larger aperture (smaller f number) or (3) decrease your shutter speed (so the lens opening, aperture, stays open longer). The exact opposite for a photo that is over exposed.
When your camera is on auto mode, it is moving these settings for you. When you are on 'aperture priority' you tell it what f-stop you want (which is how you can control depth of field too) and it will set ISO and shutter speed. If you are on 'shutter priority' mode, you set shutter speed and the camera sets ISO and aperture.
The best way to get a handle on understanding this is to set your camera in 'manual mode' where you set all three and just play around with it. Soon you will understand all they do, how they work with each other and how you can control what your end result looks like.
Last edited by MikeW
on Sun Feb 15, 2015 11:45 pm, edited 1 time in total.