Originally Posted by 19Monte78
Very simply, fill flash is typically used when photographing a subject where the background is brightly lit and the main subject (typically a person, but not necessarily) is in shadow (or is shadowed relative to the background).
Without the flash, you either expose for the main subject, which causes the background to be blown out (too bright and washed out). Or, you expose for the background, leaving the main subject too dark.
With a flash unit, you can add light to the forground subject using the flash. Then you set your exposure to correctly capture the background, and use the flash to brighten the foreground subject. Many modern 35mm SLRs will handle all this for you for typical subjects if you just set the camera to fill flash mode.
If your camera has a "fill flash" setting, just pop up the flash unit when you are shooting someone who is standing with bright light behind them. The result will make you look like a pro! Enjoy!
Regarding how to set the Rebel T1, you'll have to read the manual. I've never used one... But if you don't have a manual, and can figure out how to set different flash levels, you might try bracketing your shots - taking the same shot at various flash levels, then pick the shot you like.