EXPOSURE
In photography exposure is the amount of lighting in the image
Under exposure is when there is not enough lighting and when it is over exposure there is too much light and when it is too bright.
A way to remember them is just think when you are under a table or in a dark room and trying to take a picture it would be dark so that means that it is under exposure. To remember over exposure you can just think of sun because the sun is to bright. Normal exposure is when the lighting is just right.