Symbolism, Imagery, Allegory
We're not sure if the Pants should be a symbol or another main character—and not just because they're given the old capitalization treatment, a move that usually signifies importance.
They are so much a part of the Sisterhood, that without them, there wouldn't be a Sisterhood in the first place. But though the Pants play a major role in the story, as Carmen states, the Pants "stand for the promise we made to one another, that no matter what happens, we stick together" (Prologue. 21)—in other words, the Pants are decidedly symbolic.
Simply put, the Pants equals friendship.
Are these pants magical? They must be, since they fit all four girls fabulously. They contain Carmen's butt, give Tibby a girlish figure, somehow make Lena even more beautiful, and slide over Bridget's muscles. And the Pants don't stop there—they also somehow hide each girl's physical imperfections too.
While it's tempting to wonder where the heck we can find a pair of these jeans for ourselves, that the Pants fit each girl so perfectly clues us into something key: Just as the Pants bring out the best in each girl physically, so too does their friendship bring out the best in them more generally. But that doesn't mean it's all roses so long as the Pants are around.
As Forrest Gump would say, "stupid is as stupid does," and the girls do some pretty stupid things while wearing the Pants, so their magic must have its limits. Carmen breaks her father's window, and while Bridget isn't technically wearing the Pants during her moment of stupidity, she wears them on her way to the moment that undoes her.
The Pants don't always give the girls wisdom, that's for sure, but when they need to rebound from their mistakes, the Pants—like their friends—are there for them. And like their friends, the Pants are forgiving and they make the girls feels loved no matter what. The Pants "meant support and they meant love" (24.99)—just like the friendship between the girls who share them.
The bottom line is that the Pants are just pants (with a small "p") for Pete's sake, and they are dirty pants at that, since the girls agree not to wash them all summer. Instead of being actually magical, they produce a placebo effect—the girls feel like they are stronger, more self-assured young women while wearing them, and credit the Pants for this transformation. It's the girls themselves—and the friendship they share—that really deserves the credit in the end, though.