Arduino-CMake, as the name suggests, combines the best of both worlds:

1. Arduino
2. CMake

It is a framework that allows development of Arduino programs using the CMake build manager.

While there's nothing wrong with the Arduino IDE that comes along with the Arduino SDK, many developers feel like it's missing some valuable features other IDEs do have.

Because it's entirely cross-platform, it's modern, but most importantly - It's extremely extensible yet so easy to learn and use!

If that's your first time here, you probably should head to the Getting Started section.
If you want to dive deeper into the framework and see what does it offer, head to the Usage section.