I cannot really understand what happens to the eye in the case of glaucoma. but how's your vision with it?

Not usually: Glaucoma is often silent as you are not able to tell when there is slight damage to the visual field. Furthermore, when eye pressure is an issue, high pressure in the eye, up to about 30, can feel totally normal. No pain, no visual symptoms, or external signs may be present. Only in the late stages does the field constrict to become symptomatic. Prevention is always better so best to get exam.