Muckrakers were journalists who were trying to expose problems in American society. These problems could be social, economic, or political.
A perfect example of muckraking would be the book "The Jungle" by Upton Sinclair. This book took a look at the working conditions of workers in the meat-packing industry in Chicago. These disgusting practices used by this industry and the unsanitary conditions it resulted in caused outrage among American citizens. This demand for change resulted in President Teddy Roosevelt passing the Meat Inspection Act of 1906.
This law ensured that meat-packing companies followed certain safety and sanitary guidelines.