It is a repository that contains ready-to-use dev environments that let you work across different ROS 2 installations / distributions, and is especially useful for package maintainers that need to switch between distros often.
It provides support for:
- Melodic (base, core, desktop, desktop-full, perception, robot)
- Noetic (base, core, desktop, desktop-full, perception, robot)
Read my previous post for further details - VSCode Dev Containers for Package Maintenance across Distributions.