On Top of the World
This is a big, crowning moment in David's career: it's his Pet Sounds, his Who's Next (if you know what's up with classic rock). The Ark—which is sort of God's home on earth, or his main contact point with people—has been mobile for a really long time. David finally allows it to settle down in place (though his son, Solomon, will actually build the Temple).
Since the Ark is God's big contact point with people, it's rather volatile—a guy named Uzzah gets killed after he touches it. But, it showers someone else—Obed-edom—with blessings, convincing David that it's safe to bring it into Jerusalem. Famously, he celebrates by dancing mightily in front of it.
Lose Yourself to Dance
David's dancing is ecstatic. This isn't an uptight moment, or a moment that's all about following rules and laws—it's celebration time. Michal, David's wife, takes the opposite attitude, chastising David for demeaning himself through his dancing display in front of the people.
But David says that he's glad to be abased before God and before the common people like this. He's losing his own pride in being king as he submits joyfully to this higher power. Michal can't understand this, and so loses out on the Blessing.
This isn't just an important event for David, but important for God, too. As the writer Jack Miles observes, God calls himself a "father" for the first time after David's dance—it's as if David's dancing has managed to change God's whole relationship towards David, and also towards his people as a whole. It makes it more familial, intimate. David's name after all means "Beloved"—and since he's, specifically, God's beloved, his dance at the Ark indicates the joy he takes in this, the freedom and the power that come along with this rather elect position.