With no way to lose or get stuck, this is one of the best calming games you can download. There is always a next step, and if you can't find it - use a hint, or even skip the level.
In addition, your progress is saved after each successful move. So take a break whenever you want and come back later to continue from the same place.
You won't need to read explanations and tutorials for any of the 15 piece types, you'll learn just from playing. It starts as a super simple game with one simple idea, but unfolds into more and more interesting experience.
Another thing which makes great calming games is a good difficulty progression. The game will never feel too hard or unfair, but there will always be enough challenge to keep it fun.