What did Karl Marx mean when he called religion "the opiate of the people"? How does his view of religion fit into his thinking about modern society?

By writing that religion was "the opiate of the people," Marx meant that religion was a tool that the ruling class, or bourgeoisie, used to keep the proletariat, or working class, suppressed. Marx referred to the way that the poor turned to religion for comfort rather than starting a revolution against the upper classes, who kept them down. Like an opiate, religion offered the poor a kind of immediate comfort or release from reality, but it did not offer lasting comfort or solutions to their pain. Marx believed that only a full-scale revolution by the working class, or proletariat, could better their position.
Marx believed that modern times, including the growth of capitalism (which he wrote about in the mid-1800s), had resulted in a society in which the bourgeoisie exploited the proletariat. In capitalism, the proletariat became a commodity to be used by the bourgeoisie. The bourgeoisie had an interest in preventing the proletariat from carrying out a revolution, as this would disturb the privileged position of the upper class. Therefore, the upper class had an interest in maintaining the religion of the working class, which promised them better conditions in the afterlife, so that they wouldn't revolt in this life.