In 1492, Columbus sailed the ocean blue... only 15 years before Thomas More wrote Utopia. Yep, that's right—only recently had the possibility of discovering entirely unknown, new worlds become a very real part of More's life.
Because Utopia means "no-place" and "good-place" (see "What's Up with the Title?" for the full run-down), many people have read the book as an allegory for travel and discovery. Utopia is meant to represent our unquenchable desire to find new places that will somehow be better than ours or offer ways for us to fix our world. (This is why some people have also thought of Utopia as being similar to science-fiction, which often explores the idea that alien-contact will bring profound change for the better.) It also, of course, suggests that such a quest might be impossible.