I just finished listening to the audio version of George Eliot's novel, and will warn you now that I have basically recounted everything in the book below. Therefore if you're the kind of person who hates knowing the end of the book until they've read it, don't read past the page break because I've not blacked out most of the spoilers.
The Mill on the Floss is essentially about two siblings: the practical, unimaginative but hard-working Tom Tulliver, and his emotional, dreaming, and misfit younger sister Maggie. As tI read the story, I began to suspect each sibling might be symbolized by the two things in the title: Tom is the hardworking, stolid Mill, Maggie is the turbulent, changeable river Floss. The story opens when they're children and follows them into young adulthood, through all their family tragedies and personal conflicts with one another.