Fluid circulating inside the front portion of the eye is produced by a structure called the ciliary body, which is located behind the iris (the colored portion of the eye). This fluid circulates through the opening of the pupil (the dark spot in the center of your eye), and passes into the space between the iris and the cornea. Eventually it drains out of the eye through a mesh of tissue called the angle, the site where the cornea and iris meet.
This fluid is critical for bringing nutrients to the lens and cornea and removing waste material. When drainage channels become blocked, the increased pressure in the eye can damage the optic nerve. In glaucoma, the normal passage of fluid through the angle no longer works properly. Sometimes the fluid is reduced over time, leading to a condition called open-angle glaucoma. Other times it stops suddenly, leading to acute glaucoma.
In either case, the increased fluid pressure injures both the eye's large optic nerve and nerve fibers and causes blind spots. In the most advanced cases, glaucoma may lead to blindness.