If lit, the rear window or side mirror heaters are working. If not, there is an electrical problem.
It’s safe to drive your vehicle. Have your car checked by an auto mechanic.
Usual problems: bad electrical connection, broken wire, failed switch, failed timer component
Rear window and side mirror defrosters consist of an array of heating elements attached in very narrow strips to the glass interior. There is no danger of electric shock or burns. Window heaters may operate on a timer, usually 10-15 minutes. Activate the defroster if it is cold before using the wipers.