GFCI (or Ground Fault Circuit Interrupter) outlets cut the power if they sense it isn’t going where it should. That’s because they assume it’s going through you. You can usually find them near wet places like kitchens, bathrooms, or exterior walls.
A GFCI outlet will have buttons that say “Test” and “Reset." It acts like a breaker in your electrical panel, so you have to switch it back on when it trips. That can happen when the outlet gets wet, when it senses a power spike, or if it senses a short. GFCI-protected outlets help prevent fires and injury in each of those cases.
GFCI protection is also essential when considering exterior Christmas lights. Because you expose the lights and cords to the elements, they have a higher chance of weather damage. That means a higher chance of exposed wires and a higher chance of water finding its way in where it shouldn’t.
If you’re stringing lights along your roofline, you’re already familiar with the risk of being up that high. But using the right outlet is as important as a stable ladder when setting up Christmas lights. Using GFCI-protected outlets ensures the power will shut off if there's a problem.