The simple definition is just empty. It is used in Philippians of Christ emptying himself.
When used in reference to humans, it speaks of them not focusing on things that have eternal value.
So when people do not follow the heavenly that the Bible puts forth, but go after the earthly, they do not realize that naked they were born into this world, and naked they will leave it, with nothing. Only the things that they have prepared for eternity will be what will be waiting for them on the other side of death.