The glaucomas are a group of progressive optic neuropathies characterized by degeneration of retinal ganglion cells and resulting changes in the optic nerve head. Loss of ganglion cells is related to increased levels of intraocular pressure, but other factors may also play a role. Reduction of intraocular pressure is the only proven method to treat the disease. (Weinreb, et al., 2014; pubmed:24825645)

The fluid in the anterior chamber of the eye is being constantly replenished; intraocular pressure depends on a balance between fluid entering and leaving the eye. Fluid leaves the anterior chamber at the open angle where the cornea and iris meet; when the fluid reaches the angle, it flows through the trabecular meshwork and leaves the eye. In open-angle glaucoma, even though the drainage angle is "open", the fluid passes too slowly through the trabecular meshwork drain. Since the fluid builds up, the pressure inside the eye rises to a level that may damage the optic nerve; in open-angle glaucoma the increase in pressure is very gradual. Eventually, the optic nerve can be damaged from increased pressure and vision loss may result. [https://nei.nih.gov/health/glaucoma/glaucoma_facts]

Angle-closure glaucoma occurs when the iris is very close to the drainage angle of the eye; the iris may block the drainage angle. If the drainage angle gets completely blocked eye pressure rises very quickly, resulting in an acute attack. [https://www.aao.org/eye-health/diseases/what-is-glaucoma]