To understand glaucoma, one must understand fluid mechanics of they eye. The eyeball , or globe, has an internal pressure to keep it spherically shaped. Eye pressure is generally independent of blood pressure. Eye fluid constantly replenishes itself. The fluid is produced internally.
Excess fluid is drained via channels near the iris.
Eye pressure becomes excessively high when too much fluid is produced, or the drainage canals become blocked. High eye pressure can physically compress the optic nerve or interrupt blood flow feeding the nerve. In either case, the end result is nerve damage. This causes vision loss. Central vision is lost in severe glaucoma.