Water in dreams often symbolizes the emotions, so hail in a dream can represent emotions that have become frozen.
If you dream about hail, you may have had a traumatic experience, and you are not ready to deal with the emotional pain that this experience has caused you.
A dream that you are pelted with hailstones can be a message that denying your feelings will ultimately only increase your pain. You need to allow yourself to face your emotions and deal with them.
If you dream that you are sheltered inside a building while watching a hailstorm outside, you could be worried that someone close to you is repressing strong emotions, and you could be concerned that they will not be able to hold them in forever.
A dream that you are outside in a hailstorm but that you are only wearing light clothing can mean that you feel that others are not being open with you and expressing themselves to you, and this is making your vulnerable.