Early in the novel Tuck Everlasting, in Chapter 5, author Natalie Babbitt foreshadows Winnie's courage and growth as a character by describing Winnie's decision to slip out from her imprisonment in the yard and venture into the wood. "She did not allow herself to consider the idea that making a difference in the world might require a bolder venture." Much later, we do indeed learn exactly how Winnie makes a "difference in the world."
After meeting the Tucks, Old Tuck takes Winnie out onto a pond near the Tuck's home in a row boat and explains to her about the cycle of life; the cycle of life is an endless cycle of change to make way for new life. But since the Tucks are stuck living forever, stuck at the same age at which they drank from the spring, they are no longer part of that cycle. As a result, Tuck feels the Tucks' lives are meaningless because meaning in life comes with change and growth, as we see him explain when he says to Winnie, "Living's heavy work, but off to one side, the way we are, it's useless too" (Ch. 12). Tuck makes Winnie understand that it is essential to keep the effects of the spring a secret in order to keep the precious cycle of life going.
Once Winnie understands the importance of keeping the secret, she helps make a "difference in the world" by protecting the world from the knowledge of the spring. To help protect the world, Mae Tuck knocks down the man in the yellow suit who wants to sell the spring water, accidentally killing him. Then, when Mae is arrested, Winnie sacrifices herself by posing as Mae in the jail cell once the rest of the Tuck family have rescued her. In posing as Mae, she also helps keep Mae's true identity and nature protected, which also ensures the Tuck family secret is kept. In keeping the Tuck family secret, Winnie improves the world by protecting it.