Your intra-ocular pressure is the pressure that exists within your eyeball. This is what keeps your eye the shape that it is! It also helps to keep all the structures of your eyes where they should be. However, if the pressure in your eye gets too high, this can become dangerous. A high IoP is one of the first signs of glaucoma, although it doesn't always mean you have it!
Most people associate getting the pressure in your eyes checked with that puff of air, however this is not the only way in which it can be done!