Each number symbolizes a day of Creation, and in it is what was created on that day, as delineated in Genisis. The first day was dark and light, the second was the division between the waters of the earth and the sky, the third day was trees and vegetation, the fourth day were the sun, moon and stars, the fifth day were the birds and fish, the sixth were animals and humans, and on the seventh day, G-d rested.