20/20 vision means you can see at 20 feet what a person with normal vision sees at 20 feet. 20/40 means you need to be at 20 feet to see what a normal eye sees at 40 feet — your vision is worse. 20/10 means you have superior vision — you can see at 20 feet what others need to be 10 feet away to see.
Key Thresholds
20/20 — Normal visual acuity
20/40 — Minimum for unrestricted driver's license in most US states