The title of this book has nothing to do with dentistry. The "Floss" is actually a river. And the "Mill" in question is actually Dorlcote Mill, the family home and business of the Tullivers, conveniently located on the river Floss. A mill is basically a factory. In the early days of the Industrial Revolution (which happened in England during the eighteenth and nineteenth centuries), factories were often run by water mills and were thus called "mills." These mills produced things like textiles, grain, etc.
So this title helpfully clues us in to the setting. Or it does once you figure out that the Floss is a river. At any rate, this title is doing more than acting as a weird Google map search term. It is also cluing us into some of the book’s major themes. First off, we have the "Mill," which plays a huge role in the novel. The Mill becomes a goal for the Tulliver family, a place that they desperately want to reclaim after losing it in a disastrous lawsuit. The Mill also indicates that business and industry are going to play a role in this book. So the Mill is a home and a business, something that greatly shapes the type of life that the book’s main family lead. Secondly, we have the river Floss. Rivers and water imagery and themes play a huge role in the novel, and including a mention of a river in the title helps to highlight this fact.