What is Air Call-accept feature in Samsung Galaxy S4(GT-I9500)?

Air Call-accept is a special feature under Motion in Samsung Galaxy S4(GT-I9500) that allows you to accept incoming calls without any physical contact with your device. This feature comes under the category of Air-Gesture which is capable of controlling your device when you make specific gestures with your hand.  

 
(Representation of Air Call-accept feature)

You just need to move your hand above the sensor of Samsung Galaxy S4 (GT-I9500) in order to accept incoming calls using Air call-accept feature as shown in the image above.