It's a story of a depression-era Oklahoma family (the Joads) that loses its farm to "progress". The entire novel seems to be a warning of the dangers of naked capitalism. Economic (and natural) forces made small family farms financially untenable in the depression years. Only automation (tractors) and conglomeration paid. So thousands of families lost their farms and migrated to California to start anew.
The trip, including the selection and fitting out of a car/truck that could carry a family of 9 + dog 1000 miles for under $100 is very interesting. And the trip itself is actually lovely at times, aside from Grandpa and Grandma dying. The descriptions of the trip on Route 66 bring on nostalgia. Camping alongside the road with other migrants, the comradery and helpfulness was wonderful. Food and music were shared. Of course, many didn't make it. Some had not enough money to get all the way across the mountains and deserts to California. Local police were always on the lookout for migrants, who were always treated like vagrants and hurried on to the next town, or jailed.
Unfortunately, the capitalist system was lying in wait for the migrants, who were drawn to California by the promise of good wages for picking crops (itinerant farm work). But wages collapsed under the weight of all the migrants (unbridled capitalism always attemps to drive costs, and wages, to zero). They would work all day and barely have enough money for food, let alone gas to get to the next job. It is shocking that the local and federal governments failed these citizens so badly. Things continue to go from bad to worse. The Joads lose family members one at a time -- the family falling is apart. But there is an occasional bright spot, such as the act of complete giving by Rose Of Sharon (the girl is named after a flower) in the final act of the novel. Even when they have nothing, these people continue to give.
It's an incredible book. You must read it.