This sticker warns you that the connection is supposed to be joined with special low-temperature solder. If the temperature inside the clock gets above 120° C, the solder joint melts and interrupts the power. This prevents the clock from overheating and damaging its plastic gears if (for example) the winding mechanism should jam. A regular fuse might not blow soon enough to prevent this damage.
The problem with this is that I had no luck finding that special 120°C solder! But, I did find something else that will still provide overheating protection.