Very roughly, it is a comparitor of your eyesight against a standard of visual acuity - in this case I assume measured in metres rather than feet (which would be 20/20).
So if you are 6/6, it basically means your eyesight is as working as expected and you can clearly see a target at 6m which everybody else can see.
If your eyesight was 9/6 then it means you have been eating your carrots and can pick up said target at 9m compared to the standard 6m. However if you're eyesight is 3/6 then you need to eat more carrots as you now have to be 3m from the target to pick it up compared with the standard 6m. 20/20. 40/20 and 10/20 are, I believe, the equivalent in metres.
Alternatively, you could just look on wikipedia under eye tests or visual acuity for a better explanation. As for the nav requirement, I have no idea - I didn't think we still had them and anyway you don't need perfect eyesight to work at McDonalds
