When you look in a mirror, you see your reflection, which is a left-to-right reverse image of you. In the photo the baby is pressing his right hand against what appears to be the left hand of his twin.
Reflection is a math term that refers to the mirror image of a figure that has been flipped over a line.The line is called the line of symmetry.
Flip means to turn a plane figure over. For the diagram, figure B is a reflection of figure A. Figure B is a flipped version of figure A.
For more information and fun, see MATH:LINES OF SYMMETRY.