That's a cry for help. OR it might just be a cry for attention.
Take it seriously, though.
I have a friend who is an absolute Drama Queen. I've lost track of all the times he has mentioned killing himself. I've actually called his doctors and told them he needs help.
One thing I HAVE learned about him during the 12 years I've known him is that he is terrified of death, and goes to extremes trying to get himself treated for his hundreds of different ailments, whether real or imagined. Basically, I KNOW he is not serious. I also know that if he KNEW I had reported him to his doctors, (years ago now), he would never forgive me and would not still be my friend.
THINK about the person who is threatening, and try to think about whether or not this is something they would actually do.
IF you think he or she is serious, you should call the nearest suicide hotline and ask them how to handle it.