GPLv3 is the based software license that stands apart from Open Source and goes a step further. Sometimes referred to as Libre source. It specifically prohibits that the software be used in a proprietary program. Companies have to either license their code as GPLv3 (and provide the source code) of the program they've written, or in some cases where the code bases are separated in just a certain way, they can provide the source code to the GPLv3 licensed code (which they are only required to provide to people they've sold the software to which with freeware or not cost software would be everyone) while still keeping the proprietary part proprietary. The code base has to be set up a certain way to make that work though.

Watch a few Richard Stallman videos and you'll probably understand why people use it.

GPLv3 is one of many software licenses for open source. More precisely, GPLv3 is associated with the concept of copyleft