The explanation is quoted from Wikipedia:
The memorial includes twin walls, transecting a "gently sloped mound anchored by a granite path that is directed toward Ground Zero. Two 30-foot-high rectangular towers stretch 208 feet, 10 inches long — the exact width of the World Trade Center towers, the proportion of the walls a symbolic representation of the buildings as if they were lying on their sides. The name of each of the 746 victims is etched in stainless steel in 4-inch-high letters. A granite passsage is oriented to face the site of the twin towers. The name of the memorial is taken from the Bruce Springsteen song Empty Sky, which is about the "empty sky" where the towers once stood.