Trabeculectomy is an operation that is performed to allow free drainage of this fluid. 

Trabeculectomy One Penny Lane Liverpool Eye Clinic

The front part of the eye has two chambers. The first one is the space between the cornea (the transparent 'film' in the centre of the eye) and the iris which is the coloured, rounded part behind it. The open centre of the iris is the pupil. The second chamber is just behind the first and is the space between the back surface of the iris and the lens of the eye. 
The side 'walls' of the second chamber consists of a special type of tissue called the ciliary body. The ciliary body produces a clear fluid which is called aqueous humour. This fluid is very nutritious for the tissues of the front part of the eye and it also prevents them from becoming dry, which can affect vision. 

What is involved?
The operation is performed by having the eye numbed with a local anaesthetic injection. Very rarely a patient might need to be put to sleep with a general anaesthetic to allow him or her to have this operation comfortably. If you have a local anaesthetic, you will be awake during the operation, but will feel no pain and will not see anything, because the injection stops the eye working and you will be able to go home the same day of the operation (day surgery case). If you have the operation under general anaesthetic, you may need to stay in hospital for up to 24 hours after the operation. 

Any alternatives?
The initial treatment of glaucoma is usually with special eye drops that aim to reduce the production of the aqueous humour or help its drainage. If these do not work an alternative is laser trabeculoplasty. The laser light destroys part of the thickened wall of the channels of the trabecular meshwork and makes them wider allowing the aqueous humour to drain more freely. If the eye drops or the laser treatment fails then surgery is the only option. Laser treatment can reduce the success rate of future surgery and that is why some consultants prefer to go straight to surgery if the eye drops fail.