For most of Dancing on the Edge, Miracle's in a pretty precarious situation. It's like she's living life on a balance beam—sometimes she's steady and stable, but other times she's not. And how could she be? She's a kid with a controlling grandma with a passion for the occult, who still has to go to public school and deal with the haters there. Plus, her mom's dead, her dad disappears, and no one will tell her the truth. The book's title echoes the uncertainty of Miracle's situation; it's a shout-out to her positioning in life on the edge of fantasy and reality, past and present.
While the title definitely describes Miracle's position, though, it's also a metaphor for what it means to overcome obstacles and live fearlessly. Though doing so is super difficult—for Miracle, this ultimately means abandoning everything her family has taught her about herself, taking a leap, and trusting she can make up a dance of her own as she goes. At the end of the book, she says:
I looked down at my outfit and told myself, I will never wear purple again […] Then I thought, No, I won't say never. I don't want any rules. (30.39)
With this in mind, perhaps the title refers to Miracle's decision to stop teetering between people and their ideas for her, and instead jump into becoming her own self. Importantly, when she does, she refuses rules—which we can see as edges, or as lines drawn—and instead launches herself into a much more open existence.