What does it mean when a doctor says that you have 20-40 vision in one eye?

When a doctor says you have 20/40 vision in one eye, it means that at a distance of 20 feet, you can see what a person with normal vision (20/20) can see at 40 feet. This indicates that your vision is not as sharp as average, and objects may appear less clear or further away. It may also suggest that you might need corrective lenses for better visual acuity.