Strictly speaking, The Jungle did not reveal anything because it was a fictional account of the industry. It was not investigatory journalism. However, what the novel did do was to make people aware of real problems in the industry. Most specifically, it made them aware of the possibility that much of the meat that was processed by the industry was contaminated in the ways described in the book.
The major governmental response to this was the Pure Food and Drug Act of 1906. This law was meant to make sure that no food could be sold that was adulterated in ways that made it less wholesome. This included banning any harmful additives and the addition of any cosmetic compounds to conceal spoiled or damaged food. Both of these were types of contamination that had been described in The Jungle.