Original: In contrasts to non-permissive copyleft (reciprocal) licences such as the GNU General Public License, however, any copies and derivatives of the source code created under permissive licences may be made available on terms that are more restrictive than those of the original licence.
Simplified: In contrast, copyleft licences like the GNU General Public License require copies and derivatives of the source code to be made available on terms not more restrictive than those of the original licence.