Human vision begins with light hitting the iris and being converted to nerve signals, but there are several heavy-duty processing steps before the image reaches the parts of the brain that host consciousness and we actually "see" the image.
Any disturbance can create distorted or supplementary "vision".
And it's easily possible to imagine things, and for that imagination to be so powerful that it seems real - lucid dreaming is a well-known example.
Visions also occur in states of powerful emotion or with some illnesses.
The simple answer is that "sight" is a complex process that can easily be altered so that we see things with no objective existence - visions.
Predator