In bash, writing ${var?} instead of just ${var} or $var means if var isn't defined then bash will throw an error and not execute your command, instead of expanding it to "" and carrying on.
mv file1 file2 $subdir # oops, I overwrote file2
mv file1 file2 ${subdir?} # error message instead of disaster
My favourite use of this is for example commands in documentation, with placeholders for the user to fill in. Then it's OK if a user accidentally copy-pastes it without filling them in!
