Exposure is the amount of light a camera lets in.If it lets in to much light it is over exposed but if it doesn't let in enough it is under exposed. Minus numbers are under exposed ,and positive numbers are over exposed.It is best to stay in the middle for a perfect amount of light.
Examples of exposure
When a picture is under exposed it looks very dark,this could be good for if you want a picture to look dark or scary.It is also good for photos of silhouettes.
Examples
Here are a few photos I took to show exposure. The first photo is under exposed and the middle one is over exposed.The photo with a perfect exposure is the one at the end.