They are darkened in their understanding and separated from the life of God because of the ignorance that is in them due to the hardening of their hearts.
It is vitally important, with a verse like this, to make sure that we do not just begin ranting and raving about how ignorant the world is and how they have hardened their hearts. If you really read this verse, it should draw up within you thoughts of compassion and sadness. These people have a darkened understanding. In other words, they do not comprehend that they are missing out on the life of God.
When Jesus looked on the multitudes of people, the Bible says that he was "moved with compassion", because the people were like "sheep without a shepherd." With a true Biblical worldview, we must see with eyes of compassion.
The truth of the Bible doesn't empower us to go around saying, "we're better than you!" It should help us see that we aren't any better than anyone else, and that we are in desperate need of one who is better, one who is perfect... We are in desperate need of Jesus!
Our darkened understanding, our ignorance, may have come from the hardening of our own hearts, but the freedom from this doesn't emanate from us. There is not a one of us who was able to haul ourselves up and choose the right thing. We are a work of His.
So when you read a passage like this, pray that the Holy Spirit will move you to compassion and don't forget to thank the Father for rescuing you from that blinded life.