In the story, the judge has ruled that, as part of his probation, Luis Cintron must provide his father free labor for six months. His job at his father's salvage yard is to wash and polish all the hubcaps (wheel covers) on site. Luis doesn't enjoy his job at all, and he soon becomes moody and restless.
Part of the reason for Luis' agitation is that he misses his deceased mother. However his father, Jorge, is too emotionally invested in the past to help Luis move past his debilitating grief. The story takes an interesting turn when a beautiful young lady named Naomi Ramirez turns up. She asks the two men to help her find a replacement for one of the wheel covers (or hub caps) on her Volkswagen Bug.
In the story, the wheel cover is a metaphor for the moon. In addition to the passage provided by my colleague, here is another passage that explains the story's title:
He walked to the window, and she held up the sketch pad on which she had drawn him, not crying like a baby, but sitting on top of a mountain of silver disks, holding one up over his head.
In the passage above, the image is of Luis metaphorically catching the moon (finding the wheel cover). Like Luis' mother, Naomi has faith in Luis. After she shows Luis her drawing, he becomes inspired to look for the wheel cover. Prior to this, Luis had been unsuccessful in his search; the wheel had somehow proven to be as elusive as a full moon.
Because of Naomi's faith in him, Luis finds the wheel cover. He goes back to her house and hangs it on a branch outside her window. The text tells us that the wheel cover is "the first good thing he had given anyone in a long time."
Prior to her death, Luis' mother had been a strength to her son. She had always been the one who told him how much she loved him and how proud she was of him. After her death, however, Luis had acted out his grief in inappropriate ways. The text tells us that Luis started getting into trouble when he began high school.
On the night that Luis goes back to Naomi's house with the wheel cover, there is no moon in the night sky. Luis' success represents the overcoming of an emotional barrier. With Naomi's quiet faith in him (so similar to his mother's), Luis is able to put his mind to the task and to find what he's looking for. There is no need for a literal moon, as Luis has managed to retrieve a metaphorically elusive "moon": in finding the wheel cover, he has managed to retrieve his confidence and hope in life again, things he believed had been lost to him since his mother's untimely death.