What does "the fatness of the land and the dew of heaven" mean?
"Therefore may God give you of the dew of heaven, of the fatness of the earth, and plenty of grain and wine" (Genesis 27:28).
A "fat" land is one which produces bountifully.
The twelve spies were sent into the land of Canaan to discover several things. "How is the land, is it fat or lean? Are there trees in it or not? Make an effort then to get some of the fruit of the land" (Numbers 13:20). The use of the word "fat" to describe land is found in older translations, such as the King James Version which renders I Chronicles 4:39-40, "And they went to the entrance of Gedor, even unto the east side of the valley, to seek pasture for their flocks. And they found fat pasture and good, and the land was wide, and quiet, and peaceable; for they of Ham had dwelt there of old." More modern translations, such as the New King James Version render the same verses as, "So they went to the entrance of Gedor, as far as the east side of the valley, to seek pasture for their flocks. And they found rich, good pasture, and the land was broad, quiet, and peaceful; for some Hamites formerly lived there."
The "dew of heaven" is a reference to well-watered land that has sufficient moisture to produce heavy dewfall. When Nebuchadnezzar was punished by wandering seven years in the great outdoors like an animal, Daniel said, "Then he was driven from the sons of men, his heart was made like the beasts, and his dwelling was with the wild donkeys. They fed him with grass like oxen, and his body was wet with the dew of heaven, till he knew that the Most High God rules in the kingdom of men, and appoints over it whomever He chooses" (Daniel 5:21).
Hence, Isaac's blessing was one of hope for rich land that was well-watered for his son.