The gpl program refers to the General Public License. It is a computer software protocol certificate issued by the Free Software Foundation, and the software that uses this certificate is called free software. Most GNU programs and more than half of free software use it. The following text is a copy of the original document of the GNU General Public License of the Free Software Foundation. Linux operating system and a large number of related software are developed and released under the impetus of GPL. You will see that if you intend to modify, update or improve any software subject to the General Public License for distribution purposes, the software you modify must also be subject to the terms of the GNU General License.