The title of the book is best explained by a conversation between Francisco and Hank Rearden. Below is a quotation within Atlas Shrugged from that discussion(Part 1, page 422).
"Mr. Rearden," said Francisco, his voice solemnly calm, "if you saw Atlas, the giant who holds the world on his shoulders, if you saw that he stood, blood running down his chest, his knees buckling, his arms trembling but still trying to hold the world aloft with the last of his strength, and the greater his effort the heavier he world bore down on his shoulders - what would you tell him to do?"
"I... don't know. What... could he do? What would you tell him?"
"To shrug."
There are individuals within the novel who are the great minds and producers of the world. The efforts of these individuals maintain society. In essence, those individuals are the atlas of the society. The question posed within the book is what would happen if these people were to shrug?