This means that a person cannot see the whole scene as a normally sighted person can. This is measured in degrees. A person with a visual field of less than 20 degrees is considered legally blind. Normally sighted people have a visual field of 180 degrees in a normal eye. The circle in the middle indicates the visual field for someone who is legally blind which is much smaller than that of the normal visual field for someone who would see more of the fish.
Some people are what is called color blind.
This means that they do not see colors as most people see them. Although they are called color blind, they are not really legally blind and they can usually drive, and do things more easily than someone who is legally blind. Most blind people have a little sight of some sort or can sense light. However, their vision may make it difficult to recognize people by sight, read easily, or makes them have problems with different lighting etc. Many of them have what is considered low vision and use magnifiers, special lamps and other visual aids to allow them to use the sight they have.