It works in a way similar to a metal detector, sending out an electromagnetic signal which induces a current in materials nearby that conduct electricity, which in turns creates another electromagnetic signal that is detected by the bird; the closer the bird is to something conductive, the stronger the signal it gets back. Since the sea ice does not conduct electricity well, while the sea water does, it can determine the distance to the sea water under the ice. A laser altimeter gives the distance to the ice/snow surface, so taking the difference gives the thickness of the ice and snow.