The GPL is a software license. It's a legal document that says how you're allowed to use the software, and it's designed to protect users' freedom to run software how they want to. You can read more about the GPL and freedom-respecting software - the social movement the GPL enables - here.
GPL enforcement is the process of making sure that people and corporations respect the terms of the GPL. That means that when a company uses GPL'd software in their product and doesn't publish the source code, we work with them to correct the problem. If that doesn't work, we'll take legal action - although it rarely comes to that.
This project believes in not abusing GPL enforcement, so it will enforce the GPL in accord with the Community-Oriented GPL Enforcement Principles.
More information about complying with the GPL is available at copyleft.org.
You can add the badge to (for example) your GitHub README. Here's what it'll look like:
And here's a snippet you can use to put this in your Markdown documents:
[![GPL enforced badge](https://img.shields.io/badge/GPL-enforced-blue.svg "This project enforces the GPL.")](https://gplenforced.org)
You'll note that the badge links to this page and shows some text when you hover over it. The badge itself is SVG (i.e. a vector graphic) thanks to shields.io.
Want to do more? Donating to or helping the GPL compliance efforts of the Free Software Foundation, Software Freedom Conservancy or gpl-violations.org is a great way to contribute to community GPL enforcement.