You probably are talking about a narrow angle. The angle is referring to the area in the eye where the fluid is drained. Normally, this is open and fluid is drained so that the pressure in the eye is maintained at a normal level. When the angle is narrow, it can close off and the fluid builds up in the eye and the pressure elevates. This is called a narrow angle attack. It can instantly cause redness, pain, and blindness. The treatment when an ophthalmologist sees a patient with narrow angle is to perform a laser procedure to create an opening in the iris (called a peripheral iridectomy) so that the fluid has a way to drain even if the angle closes. This prevents the narrow angle attack. There really aren't any signs or symptoms until the angle closes and you suffer an attack. Not all narrow angles will close, but sometimes it is hard to predict. It is better safe than sorry to get the laser to prevent it from happening.
Hope this helps