This is a nice picture. Fundamentally it's about second chances and changing your life up. The title character leads a life, you could say, of quiet desperation. A string of not so great husbands, children who take her for granted, and a job that underappreciated her time and effort has left her desperate for a change. So, without a real plan, she gets on a bus and heads...somewhere.
Her accidental journey leads her to the Dakotas, where she finds a new job and a new romance. Still, leery of falling into her old routines with others, she keeps people at arms length. But gradually she finds her trust returning, and even her family life improving as her children learn to survive without her enabling their worst character traits.
This last point is a bit on the rosy side. I mean, in an ideal world leaving people to fend for themselves would end with them taking responsibility for their lives, but there are somewhat less ideal endings to that story. Even so though, I think the story is nice, and an important reminder that we aren't always as stuck in our life as we might sometimes think. Life is long (when it's not being short) and the world is big, and our lives can be what we decide to make of them.