The real legal meaning of the GNU General Public Licence (copyleft) is however it is interpreted by a judge. There has never been a copyright infringment case involving the GPL to set any precedents. Please take any discussion regarding this issue to the newsgroup gnu.misc.discuss, which was created to hold the extensive flame wars on the subject.
The legal meaning of the GNU copyleft is less important than the spirit, which is that Emacs is a free software project and that work pertaining to Emacs should also be free software. "Free" means that all users have the freedom to study, share, change and improve Emacs. To make sure everyone has this freedom, pass along source code when you distribute any version of Emacs or a related program, and give the recipients the same freedom that you enjoyed.