How do those internet balloons over Puerto Rico work?

It's been over six weeks since Hurricane Maria devastated Puerto Rico, leaving millions without power or access to reliable communication. The storm clobbered the island's communications infrastructure--after all, cell towers can't produce a signal without power, and backup batteries only last hours. The hurricane damaged the above-ground fiber lines that connect towers to the main network, too. AT&T, for example, is using stop-gap measures like portable cell towers on trucks to help get the network back up, and says that 70 percent of the population there is now covered by their network. And then there are the balloons.