Defrost is when your condenser goes through a cycle to melt the ice off of it when on heat pump heat. On cold winter days heat pumps build ice on their condensing coils. If too much ice builds on the coils, it will not work properly. This is why there is a need for the defrost cycle. Heat pump condensing units have a sensor on their coil to sense ice. Once ice is sensed it sends the condensing unit into a defrost. Basically all that is happening is that the condensing unit is put into cooling mode and the condensing fan motor shuts off causing the ice to melt on the condensing unit. Once it runs through its cycle melting the ice, it will automatically change back into heating mode.