This is not something that is unique to Ida Tarbell -- it is more like a way to relate all of the muckrakers to a bell tower... anyway, if I had to answer this question, I would say that Tarbell, like a bell tower, sounded a warning to the people around her. (In the old days, town bells would sometimes be rung to warn of danger.)
Tarbell, like all of the muckrakers of her era, wanted to alert the people of the United States to various conditions that they thought were bad for society. In Tarbell's case, she was especially concerned with the power of the Rockefellers and their oil trust. Therefore, she wrote books and articles trying to tell people about the actions of the trust.
In this way, she could be compared to a bell tower -- she acted as a warning device to alert people to danger in their society.