It relates to the workplace and environment all the characters work in. In the early sections of the book, it's life in Packingtown. Although there is hope when the Lithunia family coms to American, it is soon clear that the world of Packingtown is run by the law of the jungle. The house they buy is a major milstone around their necks, forcing them to take difficult, boring, and meaningless jobs just to make ends meet. In this Jungle, they are hunted by the real estate agent who takes advantage of their minimal grasp of English. At the end of this section, Jurgis, the main male character, attacks the house of the forman who forced his wife into a life of prostitution. He beats him badly, and winds up in another jungle --- jail.
Work presents another jungle. The meat packing plant is almost literally a jungle where animals a mercilessly slaughtered, their meat adulterated and life is disposible. Jurgis works other jobs, but either they don't last long, or he gets injured. He finally gets $100 from a rich man, but when he goes to cash this, he meets another unscrupulous denizen of the jungle to gives him change for $1.00 and swears that this is all Jurgis gave him. Jurgis assults him and winds up back in the jungle --- jail.
The next step for Jurgis is a brief life in another jungle --- politics. He is also involved in a life of crime involving political graft. Although he appears to succeed in this jungle, he runs into the foreman who forced his wife into prostitution ... this time he has the money to buy his way out of the jail.jungle, but it costs him his money and his job, and he's back wandering the jungle for a way to survive.
His possible escape comes when he finds the Socialist party, but from history we know that this is going nowhere.
Wherever Jurgis turns, he find a jungle. He lives in it, works in it, and is generally beaten by it. Wherever he turns, there is some evil that he gets involved in, and it seems that his wanderings just take him deeper and deeper into being lost in the jungle.
Upton Sinclair's "Uncle Tom's Cabin" of the Labor Movement has a title that suggests the struggles for survival that the Lithuanian immigrants had who worked in the Chicago stockyards in 1906 as they worked under horrible conditions. Also, the title is significant as the workers deal with the carnage of cattle, rats, etc. Their struggle to survive gruelling twelve-hour days of back-breaking work is analagous to animals's struggles to survive in the jungle.