Glaucoma is a group of eye conditions where the optic nerve, which connects the eye to the brain, becomes damaged. It's most commonly caused by increased pressure inside the eye.
Increased eye pressure is is usually due to buildup of a fluid (aqueous humor) that flows throughout the inside of your eye. Normally, this fluid would drain drain out by itself through a tissue called the trabecular meshwork located at the point where the iris and cornea meet. When the eye fluid is either overproduced or the fluid can't flow out at its normal rate and eye pressure increases.
Glaucoma has a strong heredity link and scientists have identified genes related to high eye pressure and optic nerve damage.