The title of the book has several meanings, but the major meaning is that given by Eddie, the young college-dropout who returns to the California barrio of his birth. Eddie can see how the people living there despair of getting further in life; he sees the entire community as infused with permanent sadness, caused by the constant inability to change their environment and situations. To cope with this despair, Eddie invents a different reason:
I had a theory about those vapors, which were not released by the sun's heat but by a huge onion buried beneath the city. This onion made us cry. Tears leapt from our eyelashes... babies in strollers... wailed for no reason. Perhaps as practice for the coming years. I thought about the giant onion, that remarkable bulb of sadness.
(Soto, Buried Onions, Google Books)
The babies crying "as practice" shows the culture's depression; there will be no need for any emotion but sadness in the future, and so the babies are getting their practice in to be more efficient in their sadness in adulthood. The onion imagery is repeated several times, and symbolizes Eddie's inner refusal to accept the real.
Another symbolism for the title is Eddie himself; he wants to change his fortunes but he always hits the wall of culture and society. Like an onion, which matures underground before being used for delicious cooking above-ground, Eddie is slowly maturing while he is "underground" in the city. He cannot reach his full potential until he is "picked," or escapes from being "buried" in the ground of sorrow, violence, and despair.