One day, Lazarus of Bethany became really sick. Lazarus was with Mary and her sister Martha, and Mary cared for Lazarus. Eventually, Mary and Martha decided to go to Jesus to tell him that Lazarus is sick. Also, Lazarus is one of Jesus's friends.
However, Jesus proclaimed that the sickness "is not unto death" but "for the glory of God, that the Son of God may be glorified through it" (11:4). But what exactly did Jesus mean?
First off, the "not unto death" talks about how death (especially the eternal physical death and spiritual death) would not be the final result of the sickness. More importantly, Jesus focused more on the glory of God, as noted since the Greek word for "for" used in "for the glory of God" (Strong 5228) has a meaning of "concerning" or "for the sake of". In particular, Jesus talks about how the Son of God would be glorified through the sickness. Jesus would use the sickness to reveal his powers in front of Mary, Martha, and even Lazarus.
Even today, bad events don't have to end in total ruin -- God can (and has) worked in the midst of bad events and used bad events to bring good through these experiences.