If you have vision 20/20, it represents that your you can see from 20 feet for objects at 20 feet clearly.
If you have vision 20/20, then you have good vision and does not mean perfect vision. It represents your vision is “Normal”
An example of vision 20/20
If Person A is standing 20 feet away from an object and see it correctly, then it is said to be 20/20. Person B cannot see the object from 20 feet, but has to come near 10 feet to see the object clearly. Then his vision is said to be 10/20, which means poor vision.
Snellen Chart can calculate Visual acuity.
Visual acuity (VA) is acuteness or clearness of vision, especially form vision, which is dependent on the sharpness of the retinal focus within the eye, the sensitivity of the nervous elements, and the interpretative faculty of the brain.