20/20 vision is an expression of normal visual clarity. if a person has 20/20 vision, it means that he can see an object clearly at 20 feet, which should normally be seen at that distance. If a person has 20/100 vision, it means that he must be close to 20 feet to see an object which a person with normal vision can see at 100 feet.
20/20 vision doesn't mean perfect vision. It only represents visual acuity. if a person can see well at a distance but is unable to see a nearer object, the condition is called hypermetropia (farsightedness). If a person can see a nearer object clearly but is unable to see an object at a distance, the condition is called myopia (nearsightedness).
A visual acuity corrected to 20/40 is normally acceptable. But a vision corrected to 20/200 or worse is considered legally blind in some countries.
The metric equivalent of 20/20 vision is 6/6 vision as 20 feet is equal to 6 meters. Metric measurement is followed in some countries including India.
20 feet or 6 meters is considered infinity from an optical perspective.