In Glaucoma, your optic nerve gets damaged and leads to a permanent vision loss. The damage happens when the internal fluid pressure of the eye increases too much resulting in damage to the nerve fibers and blood vessels in the optic nerve.
Since optic nerve fibers carry visual information, their damage affects the quality of information sent to the brain further leading to a vision loss. If glaucoma is detected at an early age, it can be controlled with little or no further vision loss. However, if you leave it untreated, first the peripheral vision and then central vision gets affected causing blindness.