The vitreous humor is the fluid that fills the eyeball. It is roughly the consistency of runny gelatin, but is made up of 98-99% of water. It is transparent and colorless, allowing light to pass from the pupil to the retina while refracting the light along the way so it focusses properly on the retina. It is produced by specialized cells in the retina.
The vitreous humor keeps the eyeball in shape and essentially presses the retina against the skull, keeping the entired eyeball in a roughly spherical shape. A loss of vitreous humor to the space behind the retina will result in a detached retina, which is something of a misnomer as the retina is not physically attached to the skull (unlike the cornea, which is anchored to the skull).
The vitreous humor is not regularly regenerated like other bodily fluids and, as such, anything that gets into the vitreous humor will stay there unless it is removed by surgery (see Lines in the Sand).