The Snellen eye chart is used to check visual acuity, which is one component of good vision. This is the sharpness of your vision and your ability to identify objects from a particular distance. The eye chart usually contains 11 rows of letters. In the first line, the letters are very large, and with each subsequent row, the letters get smaller.
During the visual acuity check, you will stand 20 feet away from the chart and read it with one eye covered. Each line is assigned two numbers: the top number, 20, refers to the distance in feet between you and the chart. The bottom number refers to the distance at which a person with normal eyesight can read the same line of letters. The last line you can read clearly is your visual acuity for that eye.