This is also known as perfect sight where no prescription is required in order to see things clearly and comfortably. The fraction 20/20 is understood as follows: the bottom half represents the distance in meters at which a perfect sighted person is able to see clearly a target seen by another at 20 feet. So if you are perfect sighted you will see at 20 feet exactly what another perfect sighted person will see at 20 feet. However, if you have trouble seeing things in the distance, you might see at 20 feet what a perfect sighted person sees at 40 feet. So this means your vision is measured as 20/40.
Assuming you have perfect sight then when you are looking at something in the distance, light from the object will focus on your retinas very precisely. When seeing something close-up the focusing is blurred very momentarily, but this is unnoticeable because your eye has an automatic refocusing ability that makes the target clear. This refocusing ability is called ‘Accommodation’ and it works by making the natural lens of the eye fatter, or more convex in shape. As we get older we lose more and more of this ability, which is why things close-up start to blur after about 40 years of plodding on the planet.
Short-sightedness means that the light focuses in front of your retinas or ‘too short’ and long-sightedness means it focuses ‘behind’ or ‘too long’.