In chapter 7 of Little Women, Laurie has just been playing chess with Jo. When Laurie leaves, Amy asks Mrs. March if Laurie is an accomplished boy. She replies by saying yes, remarking that Laurie has an excellent education and much talent.
But, Mrs. Marsh says, the great thing about Laurie is that he isn't in the slightest bit conceited about his accomplishments; in other words, he doesn't flaunt them. Amy makes the thoughtful observation that it's nice to have accomplishments and be elegant, but not be a show-off.
As Mrs. March points out, showing off isn't really necessary. If someone is truly accomplished, this will be made obvious through their natural manner and conversation. Indeed, this is precisely how Laurie conducts himself, which is why he's so "charming."
Jo then chimes in and uses a humorous analogy with clothing to further illustrate her mother's point. She says that deliberately showing off to prove that you are accomplished would be like wearing all your bonnets, gowns, and ribbons at once to prove that you have a wonderful wardrobe—in other words, it would be ineffective and ridiculous.
Just imagine a woman with a fine wardrobe who wants everyone to see what lovely clothes she has by foolishly wearing them all at once. That's what someone who shows off all their accomplishments is like.