The aqueous fluid is the fluid in the front of the eyeball. There is another compartment of the eyeball, the back compartment, which contains the vitreous fluid. The aqueous fluid is constantly produced and drained, so there is a lot more turnover of aqueous than vitreous. These fluids determine the pressure in the eye, and since the aqueous changes more, most all pressure considerations of the eye have to do with aqueous fluid. While the aqueous is supposed to stay in the front of the eyeball, sometimes it gets misdirected into the area where the vitreous fluid is. Since turnover in the back part of the eye is less than the front of the eye, this can lead to continued production of aqueous but less drainage of the aqueous, which can shoot the eye pressure up high. Thankfully, this is a somewhat rare occurrence.