The challah dough represents the prosperity Hashem has bestowed on us. When we prepare the daily bread for ourselves and our families, we think about the particular care that Hashem extends to every household and every soul. The ears of corn ripen in the fields under the rays of the sun, which is a blessing to all people on earth. The bread given to us daily is a result of Hashem’s special hashgacha.
Challah is analogous to terumas hagoren. Just as the threshing floor shows us how Hashem has blessed our fields, the dough represents the prosperity He has given our homes. Hence, what applies to the dough also applies to the wheat on the threshing floor: the owner may not use it until he separates terumah for the kohen. By taking terumah, he expresses awareness that it is Hashem Whom he must thank for the blessing of his fields, and gives it first to the kohen, who represents Him.
Challah is also intimately connected with nesachim. Nesachim teach us that the wellbeing and happiness of the nation depend directly on Hashem, whereas hafrashas challah teaches us that the livelihood of every household and every individual is under Hashem’s hashgacha. Not only is the sunlight that ripens the wheat Hashem’s messenger, but so is the income of every family. Even an individual’s share in a nation’s prosperity is an act of Hashem. This awareness is renewed every time we prepare bread for ourselves and our families. We consider the care Hashem extends to every home, and just as we separate terumah from a pile of produce, we separate a piece of dough and raise it to Hashem.
Sometimes blessing is bestowed on the people’s harvest in general, yet a man and his family can hunger for bread. Sometimes the reverse is true: in times of widespread scarcity and high prices, one man and his family do not suffer and are sustained by G-d’s generosity, even without the blessing of the harvest. Taking challah signifies that Hashem not only watches over worlds and countries, but extends His care to the cities, to homes, to the people inside them. He watches over every soul, young or old, who calls out to Him. Challah represents the special hashgacha pratis of the individual.