The exchange between Paul and his mother about "filthy lucker" is the crux of "The Rocking-Horse Winner" by D.H. Lawrence. The phrase is obviously something the boy has heard, but he does not quite know what it means. The term is "filthy lucre," and it refers to money which is usually gained dishonestly; however, Paul is confused and thinks it refers to luck.
"Is luck money, mother?" he asked, rather timidly.
"No, Paul. Not quite. It's what causes you to have money."
"Oh! " said Paul vaguely. "I thought when Uncle Oscar said filthy lucker, it meant money."
"Filthy lucre does mean money," said the mother. "But it's lucre, not luck."
"Oh! " said the boy. "Then what is luck, mother?"
"It's what causes you to have money. If you're lucky you have money. That' s why it's better to be born lucky than rich. If you're rich, you may lose your money. But if you're lucky, you will always get more money."
For Hester, Paul's mother, claims to be unlucky and, despite the fact that she has a fine home, servants, and expensive furnishings, she also has an insatiable need for more money. This conversation highlights her thinking as well as her motivation; she wants luck and she wants money. For her, the two are connected: being lucky means having money. Luck is lucre and lucre is luck.
This is the central conflict of the story, as Hester pursues money and luck at the expense of everything else in her life. Her greed overtakes he heart and there is no more room for love. Paul takes his cue from her and tries to create luck by creating money. (Ironically, of course, the money he makes would be considered filthy lucre because he gets it by gambling, but of course his mother is not deterred by how she gets any money.)When he provides her the money, she still feels unlucky because it is not enough and will never be enough.
In the end, Paul's mother is chasing money and blaming her unluckiness for not having it. In his innocence, Paul solidifies the connection between these two things for her.