What does it mean when someone double looks?

What does it mean when someone double looks? a rapid or surprised second look, either literal or figurative, at a person or situation whose significance had not been completely grasped at first: His friends did a double take when they saw how much weight he had lost.

a rapid or surprised second look, either literal or figurative, at a person or situation whose significance had not been completely grasped at first: His friends did a double take when they saw how much weight he had lost.