The numbers 20/20, 20/25, etc.
Are a measurement of visual acuity
. It has been found that a normal human eye
should be able to discriminate a spatial pattern separated by a visual angle of 1 minute of arc. This translates to a 20/20 letter being 8.87 mm high at 20 feet. If that image can be seen, the vision
is 20/20. Larger letters are in proportion. The "20" on top is the testing distance.