The title of Sinclair's novel is significant because it encapsulates his theme: that unfettered capitalism has created a "dog-eat-dog" world in which people prey on each other just as animals do in the real jungle. Some of the chief people preyed upon are the innocent and largely defenseless immigrants who arrive in Chicago hoping to build a better life through hard work.
In this work, they are largely cheated and exploited by those who mercilessly take advantage of their naiveté. Sinclair blames this on a system that sees people not as human beings but as widgets in a profit-making machine, to be used up for what money they can make for others and then cast on the trash pile.
Sinclair does not place the answer in becoming hard and savage to survive the system but in changing the system itself, advocating socialism as a more equitable system to protect the weak and vulnerable and to help them keep the profits of their hard work so that they can achieve the American Dream.