I have no idea what this game actually is, just that everybody that has played seems to think that it's the best thing ever.
It's kind of a hard game to describe, but here goes:
There are two halves to the game, combat and story. You get to choose whichever one you want to play at any given time, but you'll get locked out of progressing through one at certain points until you play through enough of the other.
Combat is a real-time strategy game with teenagers piloting mechs to fight giant robots. How did that come about? That's up for you to find out. The battles are fun and not too challenging, but the real draw here is the plot.
The story half of the game presents a non-linear narrative that gives you a ton of freedom for how you want to experience it. It's sort of like a simple click-and-point adventure game, almost a visual novel, but not quite. There are 13 main characters that each have their own story, split into chapters, and you get to choose the order you view them in, whether you want to hop around from character to character or just beeline one character's story as much as possible before moving on, it's all up to you.
Not to go into spoilers, but overall, it's a very unique experience, it's fascinating to gradually see how everything fits together. There's nothing else quite like it.