The ISO setting on your camera lets you customize how sensitive the camera will be to light. For example if you have it set on 100 that is the lowest setting, which means that is the least sensitive it will be to light. Which is perfect if you are in daylight and have lots of natural light. But as you can see below an ISO of 100 is not enough light for our image. It is underexposed. An ISO of 400 allows for the perfect exposure.
The only downside is when having a higher exposure you will see more "noise" or graininess in your image. The image below was taken from a birth I photographed. I had to use a very high ISO. Can you see the "noise" in the image?
To adjust the ISO that will vary from camera to camera. Check your manual to see how it is done with yours. Most cameras have a range of 100-1000.
Does this make sense? Questions?
Aperture tutorial is right here and shutter speed tutorial is coming next week!
I don't use flash. I am a natural light photographer. The only time during my shoots I have to use a higher ISO is when shooting a birth. (I do not shoot weddings. I definitely would see a need for flash in that situation.) Every other time I use a wider aperture (1.4-3.0) or I am outside and have plenty of light.