The inside of the front of the eye, between the cornea and lens, is filled with a watery liquid called aqueous humour. This is constantly being produced by the ciliary body and 90% of it enters the front of the eye, with 10% going into the back, behind the lens. This is not the same as tears, which are always outside the eye.
The aqueous drains through “the angle”, which is where the front of the iris meets the inside of the cornea. When the drainage is blocked, the pressure inside the eye (intraocular pressure or IOP) rises. This can damage the optic nerve and may be part of a condition called glaucoma.
After vitrectomy the back of the eye fills with aqueous humour, replacing the vitreous humour.