When a clipping plane clips the current object the light inside the button is yellow.
The Show button turns on and off the graphical representation of the clipping plane in the scene. The plane are displayed as a square with diagonals drawn in the color of the clipping plane.
The Transf button allows to bind the mouse to a clipping plane. This can also be done using the Clip Plane transformation button in the main window. The clipping plane can then be rotated and translated. The translation is always done orthogonally to the plane. Scaling will scale the graphical representation of the clipping plane.
The In/Out button is used to specify on which side of the clipping plane the object is visible.