What did the seven fat cows represent in Pharaoh's dream?

Answer: Seven years of plenty

The seven good cows represented seven years of great abundance.

“The seven good cows are seven years, and the seven ripe heads of grain are seven years. The dreams have the same meaning. Behold, seven years of great abundance are coming throughout the land of Egypt,”— Genesis 41:26, 41:29 (BSB)