what is smile back?

If you smile at a person, he/she may smile back.(你對着人微笑, 那人見了,也對着你微笑。(禮貌回應)
An example in Oxford Dictionary:
She smiled at him and he smiled back. 她衝着他笑, 他也衝着她笑。

A smiling back is a healthy, pain-free back. A smiling back is strong and vibrant. It goes with smiling eyes.Just as a person with smiling eyes enjoys being friendly and helpful because it feels good, so a person with a smiling back enjoys doing everything, even the little unimportant things, effortlessly, gracefully and supremely well — because that feels good too.But that effortless flow that feels so good disappears when your back returns to its habitual frown.
smile back =微笑

****http://www.smilingbackmethod.com/smiling-back-method.htm

Smile back simply means returning a smile.
When someone smiles at you, you smile back.

http://www.smilingbackmethod.com/smiling-back-method.htm