Here are the basic terms used in HDR photography that you should be familiar with:
Exposure can be defined like amount of light that is falling to photographic film (or chip in digital photography) during the time you are taking your photo. It is often measured and reffered like EV (Exposure Value).
Under exposure that you don’t allow enough light to fall on your film or chip during taking the photo.
Over exposure mean to much light for your photo that will usualy cause blowing of highlights area of your picture.