Looks like EW
has a better description.
In this epic adventure thriller, a family struggles to reunite in a post-apocalyptic American landscape: a world of empty cities, local militias and heroic freedom fighters, where every single piece of technology — computers, planes, cars, phones, even lights — has mysteriously blacked out … forever.
So it's just electricity or maybe more vaguely modern technology. I wonder if they will remember to kill all cheap non-mechanical watches.