[Help-bash] bash4-3 ,in configure file. help!

[Ta]

for bash4-3 ,in configure file:
if test -n "${ZSH_VERSION+set}" && (emulate sh) >/dev/null 2>&1; then :
  emulate sh
  NULLCMD=:
  # Pre-4.2 versions of Zsh do word splitting on ${1+"$@"}, which
  # is contrary to our usage.  Disable this feature.
  alias -g '${1+"$@"}'='"$@"'
  setopt NO_GLOB_SUBST




 what ${ZSH_VERSION+set} will do?
what command  the emulate is ?why i  cannot find the shell?
what setopt NO_GLOB_SUBST means?
thanks