What did Karl Marx mean when he called religion "the opiate of the people"?

Marx wrote of religion as the "opiate" of the masses in the introduction of a book critiquing Hegel that was not published until after his death. He first published the introduction in his own journal in 1844. He did not originate the idea of religion as the opiate or opium of the masses, but the phrase has become very strongly associated with him. He writes:

Religious suffering is, at one and the same time, the expression of real suffering and a protest against real suffering. Religion is the sigh of the oppressed creature, the heart of a heartless world, and the soul of soulless conditions. It is the opium of the people.
The abolition of religion as the illusory happiness of the people is the demand for their real happiness. To call on them to give up their illusions about their condition is to call on them to give up a condition that requires illusions. The criticism of religion is, therefore, in embryo, the criticism of that vale of tears of which religion is the halo.

Marx is saying that religion must be abolished because it provides an illusory happiness that prevents people from achieving real happiness. In this way, religion is like opium, sending people into a hazy dreamworld where they can forget all about their problems and feel good for a time.
However, taking opium simply makes it all the harder to face your problems; it becomes a big part of the problem. If you spend your life taking opium, you will soon be living in squalor because you won't be working, taking care of yourself, or doing anything meaningful or productive. Religion, Marx argues, is the same. Like opium, people turn to it to escape suffering, but it is not a real solution.

This phrase is taken from Marx's "A Contribution to the Critique of Hegel's Philosophy of Right" in which he talks about the role of religion in society. To put it into context, here is the quote in full:

Religion is the sigh of the oppressed creature, the heart of a heartless world, and the soul of soulless conditions. It is the opium of the people.

To understand the meaning of this line, it is worth noting that the word "opiate" is another word for a drug. Specifically, it is a drug which contains opium, and opiate drugs can be medicinal or recreational. With this in mind, what Marx is saying is that religion is just like a drug: it helps to ease people's pain and suffering and can make them feel happy and fulfilled.
For Marx, the problem with religion lay in the fact that it did not really solve people's problems. Just like a drug can offer a temporary fix for pain, religion only provides temporary relief to a problem. The problem that Marx is, of course, referring to is the problem of class conflict: the fact that the bourgeoisie continually exploits the proletariat.
Through this quote, then, Marx is expressing a negative view of religion. While he accepts that religion can make people feel happy in the short term, it does not address the long-term problems that class conflict causes in a society.