The GNU General Public License is called GPL in short. It has some terms and conditions to copy, modify and distribute the software licensed under its name.
GNU makes sure that any software source-code licensed under it has to make originating source code open and freely available to all its users. Here, freely doesn't mean by cost but it says that it is freely available to users to distribute and modify the code, but they can't impose any restrictions on further distribution, the source code has to be made available.
