Mass action in sociology refers to the situations where numerous people behave simultaneously in a similar way but individually and without coordination.
Another, more complicated example would be one based on a work of 19th-century German sociologist Max Weber, The Protestant Ethic and the Spirit of Capitalism: Weber wrote that capitalism evolved when the Protestant ethic influenced large number of people to create their own enterprises and engage in trade and gathering of wealth.
In other words, the Protestant ethic was a force behind an unplanned and uncoordinated mass action that led to the development of capitalism.
In the early books of the series, the main character, Hari Seldon, uses the principle of mass action to foresee the imminent fall of the Galactic Empire, which encompasses the entire Milky Way, and a dark age lasting thirty thousand years before a second great empire arises.
With this, he hopes to reduce that dark age to only one thousand years, ostensibly by creating an Encyclopedia Galactica to retain all current knowledge.