When you look at your grass you see a lot of imperfections. Your own grass is close to your eyes, so you can see all of its details, so you realize that not all of the grass is green.
You can’t actually zoom in on your neighbour’s lawn, because of the fence. You can only view your neighbour’s grass from far away. You can’t see the details of their grass. So, your neighbour’s lawn looks perfectly green.
This is talking about life and jealousy & envy.