Upon hearing this title, your first reaction may be, "huh?" Who is Atlas? Why is he shrugging? Well, first off, Atlas refers to a Greek myth. In Greek mythology, Atlas was a Titan. The Titans had a big war with the Olympians – Zeus and that crowd – and lost. As punishment, Zeus made Atlas forever hold up the heavens on his shoulders. In many pieces of art, Atlas is shown holding up the globe. (Check out these images for some examples.) Which brings new, literal (and painful) meaning to the phrase "the weight of the world on your shoulders."
In this book, Atlas serves as a metaphor for all the smart, creative "doers" in the world: the industrialists, the artists, the engineers, the professors, etc. Just like Atlas, these are the people responsible for supporting the entire world. In Rand's opinion, they are all mistreated and are opposed by bad government policies. So they all go on strike and basically "shrug" off the weight of the world. The shrug part is important here. This strike of heroic individuals is a nonviolent one. That's why the book isn't called "Atlas Threw Off the World."
It's also worth noting that shrug is in the past tense in the title. The novel itself is narrated in the past tense and the action covers a span of many years. This all contributes to the legend-like quality of the book's plot, and the reference to Atlas, a mythological figure, also contributes to the novel's epic scope.