Weeping & Gnashing of Teeth - Luke 13: 22 - 28

Six times in the New Testament, Jesus says, "There will be weeping and gnashing of teeth when talking about unbelievers who stand under the judgement of God and receive condemnation. What exactly does Jesus mean by it? What is the difference with those who will weep and those who will gnash their teeth? How should those of us who have loved ones who are unbelievers react to Jesus' words?