If you mean the wire inside attached to a little slider clip on the fins, that's the thermostat to regulate the temperature inside the freezer. You can adjust the temperature by sliding it up and down. Up is coldest, down is warmer. It also affects the temperature in the refridgerator, so if you're making ice cubes in trays, set it high and move it back down a bit after the ice is made or the refer may get too cold (P. 9 of owners manual).