Adama is many things, and it is hard to nail down one precise thing. However, Adama is a fundamental rethink of how to build infrastructure that brings people together. The aspiration is that Adama is simple, becomes well understood, and changes the world. This is why Adama is 100% open source.
Since "What is Adama" is hard to pin down, you can gain insights by reading Adama as a ?. Adama has a history in board games which you can read more about, and you can learn more from the requirements that drove Adama's engineering.
You can also jump in today with the CLI by following the tutorial.