The GNU General Public License (GNU GPL or just GPL) is a copyleft software license defining the terms and conditions for using, modifying, and distributing free, open-source software. The purpose of the license is to prevent GNU software from becoming proprietary.
Creators who want to publish work under GNU GPL may take other GNU GPL-licensed work and modify, distribute, and charge for said work as long as they agree to pass on the same open-source rights to the recipients.