A cold restart forces your device to start up from scratch. You will lose any unsaved work, however it may be your only way to recover from some errors.
Warning: do not use cold restart during system updates
Do not use a cold restart if your device has frozen whilst installing system updates. This could stop your device from working at all, could lose everything that was saved on your device, or even damage your device beyond repair. Instead, leave it plugged into a power supply and contact whoever supports your device or the outlet you purchased it from.