What does Genesis 6:15 mean?

In this passage, God has informed Noah of His plan to destroy the entire human race, except for Noah and his family. God has also begun to describe the "ark," a large wooden box, which Noah is to use to preserve his family and certain animals from the flood.

A cubit is a measure of length equal to approximately 18 inches, or about 46 centimeters. A cubit was traditionally thought to be the length from a man's elbow to the tip of his fingers with his hand extended. While that may sound vague to modern ears, it was not uncommon in ancient days to use body parts as measuring sticks. The concept of universal, objective measurements (such as the modern meter) was simply not practical at that point in human history.

Using 1 cubit = 18 inches = 46 cm, the craft God describes to Noah would be about 450 feet (137 meters) long, 75 feet (23 meters) wide, and 45 feet (14 meters) high. This would make the ark 1-1/2 times as long as an American football field. Or, slightly longer than a regulation FIFA soccer field. It would have been somewhere around as tall as a 4-1/2 story building. Even by modern standards, if this was a ship, it would be a big ship!

Despite the size, there's no reason to think Noah could not have constructed this ark. God nowhere orders Noah to use only his own two hands. So, as with any other project, it's common sense to assume Noah had help from his sons, and probably others, in constructing this massive vessel. This would be no different than a contractor or business owner "building" something through oversight of others.