ISO is very easy to use, what it does is it enhances your picture alot and makes it look like a picture with the flash. when i used the flash it really put alot of light into my photo, as i used the ISO the higher the ISO was the more enhanced the picture was. I would rather use the ISO then flash because when they use flash in your face u might skint or make a funny face because of the flash. But, when you use the ISO you wont skint you will look the normal. When you use a fast ISO it wont let a lot of light come into the picture.
Fill in flash is basically a cool way to get rid of shadows in a picture. When you could use fill in flash in pretty much any where, it will get rid of some shadows you have in picture and reveal your skin color instead of a shadow. The third picture is a very good example of fill in flash, look at the last one and see the difference.
A light meter measures the light in the room. It will enhance your photo's more and see the emphasis. The top picture is a good way of metering it measured more of the computer's light then anything else in the room. The bottom picture it measured my partners skin tone and made her stand out.