GNU General Public License (GPL) is a reciprocal license that protects the interests of the community by guaranteeing that derivative works of the GPL licensed code will remain accessible to the community. The use of GPL avoids the situation where the community ends up having to compete with a proprietary (closed) version of their own codebase.
GPL licensed code can be freely modified and redistributed as long as all the changes to the code remain under the same or compatible license.