It's simple, let the dead pick their own fate, meaning for the people who did not believe in jesus christ and accept him this people would be called "spiritually" dead meaning they are not saved let them pick their own fate.
I was taught that it meant to focus on the living and to break from the spiritually dead. Apparently, there are a few different meanings and interpretations: http://funadvice.com/r/bgisj1uat7p
Well he told this to someone who wanted to follow Jesus but said first he wanted to bury his father so it sounds like Jesus is saying concentrate on the living whose fate isn't yet sealed
Well i would just ask God that question when the rapture happens!