Malorie has just discovered she's pregnant as the world is ending. When her sister kills herself, she seeks refuge in a safehouse where strangers have banded together under the leadership of Tom. Thus begins a nightmare that ends where the novel begins - with Malorie blindfolded in a rowboat, frantically trying to take her children to safety in a world apparently swarming with creatures of unspeakable appearance.
The concept of seeing or reading something that drives you mad is an old one in horror fiction. Lovecraft used, it and so did Robert Chambers, whose King in Yellow was an inspiration for the HBO series True Detective. Malerman makes good use of this concept in Bird Box. We never see the creatures. They are never described. We don't even really know whether they exist during much of the novel, but that doesn't prevent them from being utterly terrifying. We don't know if they're hostile or if the effect they have on humans is purely accidental, and that doesn't matter, either.