What do the big and small letters mean on an eye vision chart?

No hidden meaning: The letters themselves aren't really the important thing-eye charts have different letters (or even shapes). The point of them is that you stand a specific distance from the chart & read what you can. The lowest line you can read tells you the "strength" of your vision, based on the numbers at the end of each line. This is a good screen, but not a replacement for regular eye doctor appointments.

Average vision: The eye chart compares your ability to see a specific size object at a determined distance (usually 20 feet) to the average person. If you can read the small letters at the 20/20 line, it means that you can see an object at 20 feet as well as the average person. If you have 20/30, it means that you have to be 20 feet from an object to see it when the average person can see it from 30 feet away.