How is the pressure in my eye measured?

Various methods: Most commonly with air puff pneumotonometry (often optometrists) or blue light applanation tonometry (ophthalmologists) where the known force indenting the cornea is canceled by the intraocular pressure pushing back which equals the estimated pressure inside the eye (indirect measurement). Other methods such as rebound tonometry, etc. Exist.

Pressure: The most common method, called applanation, uses a tonometer to very lightly contact the surface tear film-- the surface tension of the eye is converted to an internal eye pressure measurement with a clever math equation.