Mapping more than 500 million reads?

1) Failed to understand "mapping more than 500 million reads" - what is "read"?
2) Failed to understand "17.7-fold coverage of the genome"

Context:

To study the specimen, Nolan sought clues in Ata's genome. He initially presumed the specimen was tens or hundreds of thousands of years old—the Atacama Desert may be the driest spot on the planet, so Ata could have been preserved for eons. He consulted experts who had extracted DNA from bones of the http://news.sciencemag.org/sciencenow/2012/08/genome-brings-ancient-girl-to-li.html?ref=hp, an Asian relative of European Stone Age Neandertals. It turned out that their protocols weren't necessary. "The DNA was modern, abundant, and high quality," he says, indicating that the specimen is probably a few decades old.
To the chagrin of UFO hunters, Ata is decidedly of this world. After mapping more than 500 million reads to a reference human genome, equating to 17.7-fold coverage of the genome, Nolan concluded that Ata "is human, there's no doubt about it." Moreover, the specimen's B2 haplotype—a category of mitochondrial DNA—reveals that its mother was from the west coast of South America: Chile, that is.

MOre:
http://news.sciencemag.org/health/2013/05/bizarre-6-inch-skeleton-shown-be-human