It refers to opium, a drug.
Religion is a drug. It can totally confuse you and lead you off track. Religion tells yout to work your butt off to earn your way to heaven. It is completely meaningless, and yes, it is empty. This is why:
In reality, you will never get to heaven by doing stuff, because God's benchmark for getting into heaven is perfection!
Christianity says we will go to hell because we've sinnned (doing wrong stuff such as lying, stealing, hurting each other, etc.) However, God loved us so much, he didn't want us to suffer, so he came to earth as a human in the form of Jesus, and because he was perfect, he was able to die on the cross and take our punishment on himself instead. Think of it this way: we've broken the law, and we have to take the consequences and go to jail. But if someone steps in and pays the bail, we can go free! In the same way, Jesus paid dearly for the sins that we committed, because he loved us. This means that our bail money to God is paid and we can go free!
We don't need to depend on religion (doing works to get to heaven) to go to heaven, because the path has already been laid! Instead, we just need to turn to God, say sorry, and accept the 'bail money' that Jesus paid for us!
It's that simple, yet religion makes it so hard.