The picture does a good job of summarizing what it is, but I'll go into some more detail.
Glaucoma is a semi-rare eye disease that causes a buildup of pressure behind the eye and damaging the optic nerve; depending on what type of Glaucoma this can take several years, decades, or hours.
There are various treatments if it's bad enough, which begin with just medicine and end with surgeries that carve tunnel through your eye to help with drainage.
I'm twenty and have lived with an extremely severe case for thirteen years. I'm post bypass surgery, but I've started having my migraines again and no longer have insurance; let's have a thread before I lose vision in my left eye!