What did Marx mean when he said that "religion is the opium of the people"?

Well firstly, we need to understand what opium is... It was a powerful drug (similar to today's heroin), that was really popular during Marx' time. Opium was used in small doses (for things like medicine, and painkillers) to large ones (allegedly, the Turkish Empire gave opium to their soldiers to reduce fear and tiredness in battle). As a narcotic, opium enhances your mood and confidence, and can mask feelings of pain or misery - but only during the 'high'. I'd recommend researching the side effects so you really understand to what he was referring. Now when he speaks about "the people" remember that Marx usually means "the proletariat" - who spend their lives being oppressed and exploited by the bourgeoisie. He was suggesting that religion is their 'drug': it numbs the pain of daily life by suggesting that there is a God listening, and a better life after death. It also gives an outlet for their pain in the form of things like worshipping, or singing songs together - so, like opium, becomes highly addictive. Remember as well that Marx believed that religion was controlled by the bourgeoisie: so they gave this 'drug' to the people, to create a state of artificial hope and confidence, and avoid them rising up to revolution.