The narrator of The Jungle focuses largely on Jurgis Rudkus and his feelings, but we also get some insight into the thought processes of the less important characters. The narrator moves relatively effortlessly into the minds of everyone in the book. However, while this third-person narrator sees all and knows all, he is not totally objective in tone. Take, for example, this passage early on in the novel, as Jurgis's family approaches Packingtown for the first time:
A full hour before the party reached the city they had begun to note the perplexing changes in the atmosphere. It grew darker all the time, and upon the earth the grass seemed to grow less green.[…] And along with the thickening smoke, they began to notice another circumstance, a strange, pungent odor. They were not sure that it was unpleasant, this odor; some might have called it sickening, but their taste in odors was not developed. (2.12)
First of all, we find this passage interesting because it shows that the business of the slaughterhouses has a subtle influence on the very geography of the land around them. The unhealthy atmosphere of these places is killing the grass and filling the air with "a strange, pungent odor."
The narrator strongly hints that even the atmosphere of Packingtown is bad, with its dark smoke and smell that "some might have called […] sickening." So we know that something is rotten here; we can see foreshadowing that Packingtown is going to have an evil influence on Jurgis and his family. The characters themselves "not sure that it was unpleasant," though, because "their taste in odors was not developed."
In other words, Jurgis and his family are so utterly naïve that even the smell of the stockyards – where ten thousand cattle are killed every day – doesn't immediately strike them as unpleasant. The narrator is sharing foreshadowing with us, the readers, but the characters are too ignorant (and unfamiliar with the setting) to notice all of these hints of future misery. This is why we say that the narrator is omniscient but not objective: he knows what is going to happen to these characters but the characters themselves do not. The narrator is trying to excite our pity by demonstrating how clearly doomed Jurgis and his family are. Like the cattle going passively to their fates on the killing floors, these people are heading straight to their own destruction, and the narrator is trying to make us feel bad about their fates.