Hi Mae!
Good Eye can be interpreted in many ways. Firstly, it can be used in archery were you see the target with one eye.
However, it can also be a complement - say you point out something in the distance, your friend could say to you - "Good Eye!" You saw something far away.
Also if say your friend has a new belt (or other clothing item) and you notice it and point it out. They could say "Good Eye" too.
Hope this helps you! Have a great weekend :) - Brad