Can anyone tell me what is Glaucoma?!

Think of your visual system as a Security Camera System.

Security camera = eye ball
Wires from the camera to Computer monitor = optic nerve
Computer montor = brain cells

Now for some reason, the wire that is joined to your camera... exactly at the join.. is damaged... So although your camera can 'film' what's going on... it can't send the complete picture through to the computer.. and so the computer only gets a partial picture.

So in Glaucoma, the optic nerve is damaged somehow... usually by lack of blood supply... The pressure (that keeps the eyeball 'pumped-up) can get so high that it mechanically pushes (an crushes) the optic nerve, or stops the blood from nourishing the nerve.

Now in treatment, the pressure in the eye is dropped by drugs or by surgery... or the blood that supplies the nerve is increased... or both..

Glaucoma is an eye disorder in which the optic nerve suffers damage, permanently impacting vision in the affected eye(s) and progressing to complete blindness if untreated. It is often, but not always, associated with increased pressure of the fluid in the eye (aqueous humour).[1]
The nerve damage involves loss of retinal ganglion cells in a characteristic pattern. There are many different sub-types of glaucoma but they can all be considered a type of optic neuropathy. Raised intraocular pressure is a significant risk factor for developing glaucoma (above 21 mmHg or 2.8 kPa). One person may develop nerve damage at a relatively low pressure, while another person may have high eye pressure for years and yet never develop damage. Untreated glaucoma leads to permanent damage of the optic nerve and resultant visual field loss, which can progress to blindness.
Glaucoma can be divided roughly into two main categories, "open angle" and "closed angle" glaucoma. Closed angle glaucoma can appear suddenly and is often painful; visual loss can progress quickly but the discomfort often leads patients to seek medical attention before permanent damage occurs. Open angle, chronic glaucoma tends to progress at a slower rate and the patient may not notice that they have lost vision until the disease has progressed significantly.

Glaucoma is an eye disorder in which the optic nerve suffers damage, permanently impacting vision in the affected eye(s) and progressing to complete blindness if untreated. It is often, but not always, associated with increased pressure of the fluid in the eye

wikipedia

when eye pressure is over 21mm of Hg its called glaucoma .that is a rough figure ...it may vary individually .this causes progressive loss of vision if pressure is not controlled /brought down by surgery or eyedrops .

Is an eye disorder in which the optic nerve suffers damage, permanantly impacting vision in the affected eye, and progressing to complete blindness if not treated

Wikipedia