What do winemakers mean by first, second and third "fill"?

Hello there! I'm Dr. Vinifera, but you can call me Vinny. Ask me your toughest wine questions, from the fine points of etiquette to the science of winemaking. And don't worry, I'm no wine snob—you can also ask me those "dumb questions" you're too embarrased to ask your wine geek friends! I hope you find my answers educational, empowering and even amusing. And don't forget to check out my most asked questions and my full archives for all my Q&A classics.
Dear Dr. Vinny,
What do winemakers mean by first, second and third "fill"?
—Nambitha, Cape Town, South Africa
Dear Nambitha,
I’ve heard these terms used in reference to wine barrels. When you have a brand new barrel and fill it with wine, that’s the “first fill,” second time is the “second fill” and so on. So when you hear winemakers talk about a wine being barrel aged in "new oak," that would be the first fill.
Why is this important? The intensity of a barrel’s impact depends on how new it is. A barrel’s first fill will impart the most intense spice, cedar, tobacco, caramel or vanilla notes. Each subsequent fill will be less potently affected. A barrel that has been used a few times is typically considered a “neutral” barrel.
—Dr. Vinny