I think it's about these two people who live in a village that's covered my a big mechanical 'umbrella', and the rain is actually leeks from the machinery. They've been told story's about a land with blue sky's and go searching for it. The shadows they see along the way may be ghost of people who went searching for it as-well
However, some sources say it is about Global warming .
Their "sleep" at the end and the "umbrella" didn't make much sense until I stumbled upon one comment that suggested that the umbrella was actually an old broken down nuclear reactor that had released a giant umbrella shaped cloud of radioactive gas. When they fell asleep at the end, it was actually death from exposure.