Answer:
I understood this quote to mean that 'religion' in the general sense keeps the people listless and sedated as an opiate or opium leaves people. Opiates can cause a person to sleep their life away, not caring about eating, working, or even their loved ones, eventually sleeping themselves to death. But paradoxically, opiates can also alter the consciousness of the users, and free them from religious dogma. Marx seems to be implying that people should dedicate themselves to anarchy and to fighting social and authoritarian injustice.
Religion means a "set of rules", rules to follow in order to guarantee an afterlife. It's used by the people in power to control the people who are not in power (the masses). It convinces them to accept and endure the suffering and injustices in this life because heaven awaits them. It prevented the masses from questioning and ultimately changing the current ways so the people who were in power could stay in power.