"Just" intonation is tuning based on the notion that ratios of frequencies should be ratios of simple whole numbers. That is a complicated way of saying: the distance between notes in a scale should be simple. And it turns out the ear agrees: "just" intervals sound more pleasing than what your $20 digital tuner or your cell phone app will lead you to tune.
We'll explain a simple way you can get most of the benefit of a just tuning.
Oh, and 432 Hz? We'll talk about that too. While I understand 432 has some avid proponents, I myself do not perceive any musical or therapeutic benefit to a 432 Hz reference compared to a 440 Hz reference, but we'll explain how to do that too.