While reading "The Elegant Universe" by Brian Greene, I got stuck on the chapter about quantum geometry where it suggested that there are apparently 2 concepts of distance. I recall something about measuring distance with a probe traveling across the distance at a fixed and known speed and measuring how long it takes for it to cross the length. With the time and the speed known, the distance can be calculated. However, the book said something about using strings wrapped around a circular dimension and strings that are not as a probe, which gives two different notions of "distance" in our universe. I do not understand how does wrapped and unwrapped strings give two different notions of distance. Can someone explain it to me?