Steinbeck wrote The Grapes of Wrath during the Great Depression to critique what he considered the failures of the capitalist system. A radical at that time, Steinbeck did not believe the New Deal or the Democratic Party went far enough to address systemic injustices in US society.

In writing "the grapes of wrath are filling and growing heavy ... for the vintage," he is using a metaphor that compares the wrath or anger of the many poor people in the United States at that time to grapes growing on the vine. Just as grapes come to fruition by being exposed to water, sun, and nutrients in the soil, the "grapes" of the anger of the common people are being fed by unemployment, hunger, unfairness, and lack of opportunity. The implication is that the "fruit" of this suffering will be an uprising or revolution.

This statement is based on fears at the time that capitalism itself had failed. The aftermath of the stock market collapse of 1929 had shown that the private sector was unable to self-correct on its own and steer the economy back on course. The Russian Revolution was only twenty years old when Steinbeck wrote his novel, and people were well aware that economic collapse had, in part, sparked that revolution. Steinbeck, in his statement, is openly playing on the fears of the middle and upper classes, encouraging them to change the system or end up with a revolution.

19 So the angel thrust his sickle into the earth and gathered the vine of the earth, and threw it into the great winepress of the wrath of God. 

20 And the winepress was trampled outside the city, and blood came out of the winepress, up to the horses’ bridles, for one thousand six hundred furlongs.

Revelations: 14:19-20

A Biblical allusion as well, the title, The Grapes of Wrath recalls the final acts of God for justice. Said Steinbeck himself:

I want to put a tag of shame on the greedy bastards who are responsible for this [the Great Depression].

The oppressed such as the Okies are "ripening" in their understanding of their oppression. The fruit of their anger is ready to be harvested--the "grapes of wrath" are fully ripe. At the end of Chapter Twenty-Five, harvests of potatoes are thrown into the river, and crates of oranges are dumped after being sprayed with kerosene. And,

in the eyes of the hungry there is a growing wrath. In the souls of the people the grapes of wrath are filling and growing heavy, growing heavy for the vintage.

Greatly angered at the purposeful spoilage of food in order to drive prices up, the oppressed migrant and other migrant workers, then, begin to organize and effect justice. This growing anger at their disenfranchisement is what Steinbeck felt would foment the lower classes to revolt against their capitalist oppressors.

Thought of by his wife, the title was approved by Steinbeck because the novel itself, he declared, is a "kind of march" as it is in the American tradition of revolution. In fact, Steinbeck's short lyrical chapters of exposition are punctuated with the narrative chapters in a short one, longer two beat. The intercalary chapters were called "pace changers" by the author and were designed, he said,

"to hit the reader below the belt [because] with the rhythm...of poetry [which can]--open him up and...introduce things...which he would not or could not receive unless he were opened up."

As Steinbeck himself said, The Grapes of Wrath is, in tone and scope, "symphonic." It is, indeed, like the words of the "Battle Hymn of the Republic" and the powerful passage from Revelations, both impressive and stirring.