It argues that since mass media news outlets are now run by large corporations, they are under the same competitive pressures as other corporations. According to the book, the pressure to create a stable, profitable business invariably biases the kinds of news items reported, as well as the manner and emphasis in which they are reported, and these biases occur despite the best efforts of the reporters and editors to fairly report all the newsworthy events.
The book further points out issues with the dependency of mass media news outlets upon major sources of news, particularly the government. If a particular outlet is in disfavor with a government, it can be 'shut out', and other outlets given preferential treatment. Since this results in a loss in news leadership, it can also result in a loss of readership/viewership. That can thereby result in a loss of advertising revenue, which is the primary income for most mass media (newspapers, magazines, television).
The threat of lost revenue can result in outlets reporting news in a tone more favorable to the government and giving unfavorable news about the government less emphasis.
The documentary film Manufacturing Consent - Noam Chomsky and the Media was based in part on Chomsky and Herman's work; the remainder of the film serves as a biography of Chomsky.
See also: corporatocracy, Media Imperialism