There are a million ways to explain exposure. In my experience, working with artists like ourselves, we tend to learn in the simplest fashion. So simply put, exposure is the amount of light in your picture. If its too bright, its overexposed. I know the photographers out there are cringing looking at that definition but for people new to photography, its simple and close enough to the point. There are 3 main factors that go into the exposure of an image. Those factors are: aperture, shutter and iso. When doing a shoot I like to start with my iso.
Iso is basically how sensitive your camera is to light. Iso can save your pictures in a lot of low light situations but can also make your picture over exposed and grainy. I like to keep my iso as low as possible at all times. While using an external flash, in high light environments I like to keep my iso around 100-400. When it comes to dark environments like a bar scene or a movie theater, I could bump my iso up to as much as 3400.
Next I like to set my shutter speed. Shutter speed is kinda like how fast your camera captures the moment. If I am shooting people posing for pictures I like to keep my shutter speed around 1/200. Your shutter speed should be as low as possible without making the picture blurry. If you are shooting something high speed your going to want to bump your shutter speed up. So if your taking pictures of someone dancing or running make sure you shoot closer to 1/300.
Aperture is usually the one I am changing most frequently during shoots. Aperture is the amount of light your lens lets into the camera. The higher the aperture the lower the light comes in the lens. Think of a circle slowly closing while letting in less and less light as it gets smaller. This one varies when your out on a shoot. You want to shoot smaller apertures in brightly lit situations which will allow less light and avoid over exposing the photograph. While shootings stuff like the stars, your going to want to open that lens up and let the light in.
These are the 3 factors that go into not making your pictures too bright (over exposed). Also to ensure your capturing the photo you want. Each moment is captured strategically when taking pictures and you have to see the future. Know what your going to shoot. Anticipate what the lighting and speed of the subject. All these things will help you to become a better photographer.