Pharaoh’s dreams are of seven ears of corn, and seven foreign cows invading Egypt and gobbling it up. The dream, like the Torah itself, can be understood in a variety of ways – not merely the way Joseph interprets them (years of plenty and then famine).
I personally favor the understanding that these dreams were of the 70 (7×10) Israelites coming into Egypt, and then devouring the land – the dream was a message to Joseph that his family would end up triumphant, even though they entered the land looking like foreigners, and undernourished foreigners at that.
The word used to describe the thin corn and gaunt cows is, in Hebrew, “dak.” It appears in the Torah here for the first time. And when Pharaoh describes the ears and cows, he takes pains to point out that even though they consumed the Egyptian grain and bovines, they remained unchanged in the process!
The second episode in which “dak” is found is in the description of the manna:
When the fall of dew lifted, there, over the surface of the wilderness, lay a fine (dak) and flaky substance, as fine (dak) as frost on the ground. Ex. 16:14
The manna comes as the culmination of the dreams themselves – those who came into the land as “dak” were sustained as they left the land with food described in the same way. The “dak” nation was unchanged in this characteristic from before and after Egypt. The Manna fulfilled Pharaoh’s dream.