In "God Sees the Truth, But Waits" by Tolstoy, what is the meaning of Aksionov's wife's dream?

Ivan Aksionov's wife's dream is a premonition of the terrible fate that awaits him. In her nightmare, she dreams that when he returns from the fair, he will have gray hair. This could be interpreted as meaning that Ivan will be separated from his wife, and will only be able to return to her when he's old and gray. That's certainly how his wife interprets the dream. She appears to be a deeply superstitious woman, fervently believing in the power of dreams to foretell certain events, especially if they happen to be bad.
Her understanding of life is simpler, more primitive than that of Ivan, who's very much a man of the world. The folk-wisdom exemplified by her interpretation of the dream is contrasted with Ivan's more worldly wisdom derived from his career as a successful merchant and businessman. In her much simpler view of things, Ivan's wife has gained a greater insight into the truth than Ivan, despite—or perhaps because of—his active participation in the world.

In the story, Aksionov's wife dreams that Aksionov returns from town with grey hair. She begs him not to leave for the fair, but he ignores her warning.
Her dream foreshadows that something evil will befall Aksionov in town. For his part, Aksionov tries to laugh away his wife's fears. He tells her that she's only worried he'll spend too much at the fair; then, he tries to placate her by arguing that the dream is actually a "lucky sign" that he'll sell all his goods and bring some presents back to her. Aksionov leaves for the fair and half-way to town, puts up at an inn for the night. During the night, a fellow merchant has his throat slit; the next morning, Aksionov is accused of the crime and of stealing twenty-thousand rubles from the merchant.
Even though he is innocent, Aksionov is flogged and sentenced to twenty-six years of hard labor in Siberia. Before he leaves, Aksionov's wife visits him in prison, and she again reminds him of the dream that she had. She maintains that if Aksionov had heeded her warnings, he would not have been caught up in such a predicament. So, Aksionov's wife's dream is a foreshadowing of the trials that Aksionov will endure, trials that eventually age him before his time.