When you drive, do you tend to notice the cars following and passing you by on the side of the road? You would be surprised that some people cannot see those cars. There are people in the world who do not use their peripheral or "blind" area.
The blind-spot monitor is a system in which, when you indicate to change lanes, your side mirror will show the view of cars near your vehicle. If there is a car in your blind spot, the door handle and side mirror will light up, warning you to look at the mirror and act.
The primary purpose of a blind-spot monitor is to prevent accidents that occur due to blind spots. Newer car models on the market today may have additional blind spot alarms, which will flash and beep to alert you when a lane is clear to change.