How accurate is the find my iphone app?

Can it be wrong by 10-20meters? What does the green circle mean?

It can be wrong. It will not show you a exact location. It will show you the radious. Consider it a lost!

It can be off by 3-10meters and the green circle is the radious

10 meters is small so yes it can be off by that, but its very accurate. It shows mine exactly where its at when I check it.