What does a pressure number in glaucoma actually mean?

How inflated... ... The eye is. Pressure is measured in milimeters of mercury (mmhg). Most people have eye pressure between 10-20 mmhg. If it appears higher (especially if there is a family history of glaucoma), your doctor may want to perform some tests to rule out the disease. Pressure is also one of the few tests we can use to see how well our glaucoma treatment is working.
Internal eye pressur. The eye pressure (IOP) is measured in mmHg and indicates the internal pressure of the eye. It's influenced by corneal thickness and fluctuates over 24 hours by about 5mmHg normally. The range is generally 10-21 on average. Eye diseases can influence IOP & the susceptibility of the optic nerve to any given pressure. The pressure is a guideline but it's affect on eye health has to be evaluated by MD.
Mm HG. Technically it is mmhg. It is the intraocular pressure measured in millimeters of mercury. It is created by fluid produced in the ciliary body of the eye.
Estimation. The pressure number in glaucoma is an approximation of the true pressure inside the eye -- most methods are measuring the pressure outside through the cornea. This number can fluctuate just like blood pressure, and changes with positioning, valsalva, heartbeat, time of day, etc.