This serves to provide a succinct overview of the GPL.
Wikipedia serves as a nice overview of what the GPL is, so here is their summary taken from: https://en.wikipedia.org/wiki/GNU_General_Public_License
The GNU General Public License (GNU GPL or GPL) is the most widely used free software license, which guarantees end users (individuals, organizations, companies) the freedoms to run, study, share (copy), and modify the software. Software that allows these rights is called free software and, if the software is copylefted, requires those rights to be retained. The GPL demands both. The license was originally written by Richard Stallman of the Free Software Foundation(FSF) for the GNU project.