On ne voit bien qu'avec le coeur. L'essentiel est invisible pour les yeux.Wikiquote offers a number of English translation but they all come up short.
It is only with the heart that one can see rightly; what is essential is invisible to the eye.
one sees well only with the heart. The essential is invisible to the eyes.
The essential things in life are seen not with the eyes, but with the heart.
My preferred translation would be rather a loose one:
You can only really see with the heart. What matters most is hidden from the eye.