In his younger years, Forrest needs to wear leg braces because, in his doctor's words, "his back's as crooked as a politician." At first, these leg braces are an outward symbol of Forrest's profound difference, even though his momma tries to convince him otherwise: "If God wanted everybody to be the same," she says, "he'd have given us all braces on our legs."
Eventually, though, the braces switch from a cage symbolizing Forrest's limitations to an almost magical symbol of his ability to triumph. When Forrest is running away from bullies one day, his braces break away from his legs. As Forrest remembers, "I can run like the wind blows."
Et voila: Forrest overcomes a weakness and turns it into a strength, just like he does with his intelligence. Unlike the movie's more unfortunate characters Jenny and Bubba, Forrest can break through any disadvantages he might have been born with. Ah, privilege.