An eye chart is an instrument used to measure visual acuity, that is how well you can see in the distance. A typical eye chart looks like this:
as you can see, the chart features various lines of test letters with different sizes. The top line has only one letter, which is the biggest, and the other contain letters that become smaller and smaller.
When you go to the doctor to get your vision tested, he will probably point out at letters (usually the very small ones) to see how far down the chart you can correctly read. Obviously if you can read the bottom one, it means your vision is pretty good.
There are variations of these charts for people who are illitterate or for kids who cannot read which use simple pictures or patterns instead like this one:
here the patient would have to indicate where each broken piece of the circle is located although these types of tests can be less accurate than the first one.
Hope this helps!