What this vision test measures
Visual acuity is how clearly you can see fine detail from a distance. It's the first thing an optometrist checks, usually with a letter chart on the wall. This test uses a tumbling E: the letter E points up, down, left or right, and you say which way. You don't need to know the alphabet, and guessing can't trick it.
Each letter is drawn at its true clinical size for the distance you choose, from 20/200 down to 20/10. That only works if the site knows how big your screen's pixels are, so the test starts with a 20-second calibration using any bank card.
What does 20/20 vision mean?
20/20 means that at 20 feet (6 meters) you can see what a typical healthy eye sees at 20 feet. In metric countries it's written 6/6. A score of 20/40 means you need to be at 20 feet to read what most people can read from 40 feet away, so your vision is less sharp. 20/15 or 20/10 is sharper than average.
Many driving licenses require about 20/40 or better in at least one eye, with glasses or contacts if you wear them. In the US, 20/200 or worse in the better eye with correction is the threshold for legal blindness.
How to get an accurate result
Wear your usual distance glasses or contacts. Stand as far back as your room allows, ideally 3 meters (10 feet), and answer out loud or with the arrow keys. Make sure the room is well lit and the screen brightness is up. Cover one eye fully with your palm, without pressing on it.
What your result means
If one or both eyes score worse than 20/30, or the two eyes are noticeably different, it's worth booking an eye exam. Blurry distance vision is most often caused by nearsightedness (myopia) or astigmatism, both easily corrected with glasses or contacts. A sudden drop in vision is different: see a doctor the same day.