Aqueous humor is produced by the ciliary body in the posterior chamber. It circulates to the anterior chamber, through the pupil and leaves the eye through the trabecular meshwork.
A narrow angle means the iris is pushed forward, blocking the trabecular meshwork.
There is also a 'pupil block', where aqueous humor fails to pass through the pupil and drain out through the trabecular meshwork.
The resulting, often severe elevation of intraocular pressure can damage the optic nerve (glaucomatous optic neuropathy) causing severe and permanent vision loss.