being untouchable means you outshine anyone who tries to come your way. no one can fuck with you because they'll bite their own tail trying to get to you. you're better, you're superior. thus, being "touched" means someone gets to you. the general message of the song is that youngsters who feel untouchable should chill, because they can indeed be touched, and even though the expression could mean different things in different contexts, i guess it's safe to say that in this track it means being killed.