Intraocular pressure (IOP) is the pressure inside your eyes. Normally, this is around 16 mmHg (2.1 kPa). Pressures greater than 21 mmHg (2.8 kPa) are considered elevated and could signal glaucoma or iritus. Elevated IOP is a bad thing because the increased pressure can cause permanent damage to the optic nerve.
An ophthalmologist or optometrist will measure intraoccular pressure using a tonometer. One kind of tonometer pushes against the pupil to determine the pressure inside, while your eye is anesthetized. Air tonometry, by contrast uses a puff of air for a faster, but less acurate measurement.
The pressure inside your eye is controlled by the production of aqueous (fluid) inside the eye which then filters "from the anterior chamber through the trabecular meshwork"1.
1. http://www.stlukeseye.com/eyeq/IOP.asp
http://www.glaucoma-foundation.org/know.htm
http://www.medicina.univr.it/glaucoma/Clinica_Di_Verona/clinic/pression.html