Some people worry about free will. They worry in particular about not having it.
If our universe is deterministic, a hypothetical being who knows all the physical properties of the universe at one point in time - where all the particles are, and where they're moving - and possesses sufficient computing power, knows the entire history and future of the universe. He knows, for example, what you're going to choose for breakfast tomorrow - and the day after, and the day after that, and all your future breakfast choices until you die (and he knows the date and cause of your death).
If the whole world, including our brains, works like clockwork then, I may worry, "I'm" not really deciding anything I think or do. The laws of the physical universe have decided everything already: my initial makeup, my external environment, and how I will respond to it. So how can we be responsible for what we do, and how can we be praised or blamed?
Feeling uncomfortable about this, but unwilling to abandon a universe ruled by physical laws, some have argued that we might be able to escape the problem if our universe is not deterministic. Happily enough, quantum mechanics shows indeterminacy exists in the physical world. That is, reality appears to have a degree of randomness about it, which appears mainly at the subatomic level. There are many possible future universes, and so even an omniscient being doesn't know for sure what the future looks like.
But is this relevant to how much control you have over your own decisions? Dennett argues, though it is an aside to his main thesis, that it does not. If what happens in the universe at a subatomic level is based entirely on impersonal physical processes, "you" don't obviously have more free will if those processes are non-deterministic.
Dennett's view seems to be that all attempts to argue that what happens in your brain is not the result of impersonal subatomic interactions seem to involve postulating explicitly or - more commonly these days - implicitly, some kind of immaterial soul or mind that is distinct from your body (the idea known as Cartesian dualism). "You" and your consciousness are separate from your physical brain, at least at critical decisonmaking junctures, and so you're not entirely bound by what happens in it, and can exercise "control" over the direction your brain's processing takes.
Dennett refutes arguments that try to derive free will from indeterminacy, in particular those of Robert Kane. Dennett sees free will at a subatomic level as both unattainable and - equally provocatively - not even desirable.
Dennett is by no means a skeptic about free will. The first main point of his book is that morally significant free will, the kind that most if us want to have, has got nothing to do with what happens at a subatomic level, or whether or not our universe is deterministic. "Free will" exists at a higher level of abstraction. This is called the "compatibilist" version of free will, held by many philosophers from Hobbes and Hume onward.
If you zoom right in on me, or on you, we're just an assembly of particles behaving according to physical laws. If you look at in any particular neighbourhood of molecules, you can't tell if you're looking at a human being or a lump of coal. But as you zoom out, you start to see patterns and structures - cells, tissues, organs, and eventually animals. What's more, those structures are not just a chance grouping at one instant: they are persistent, and self-replicating and evolving over time. Dennett holds that it is at this level that notions of avoidance, will, and choice emerge. They are innate "designed" capabilities.
We are natural born choosers. We constantly receive information from the environment, process it (both "consciously" and otherwise) and then make decisions to cause particular things to come about, or to avoid things from coming about - to the extent that we foresee or anticipate them. Many animals, in fact, exercise some degree of choice, but we have evolved this capability to an extremely sophisticated and qualitatively greater extent. In Dennett's example, if we're at bat in baseball and the ball is pitched at our body, we may choose to avoid it to escape pain and injury (as many animals would) or we may avoid avoiding it in service of some other uniquely human goal we have in mind (gaining a walk to first base, winning the game, etc).
At this level, we have a kind of free will, so long as no one else is actively coercing us to do one thing or another. Dennett, in common with other compatibilists, thinks this everyday version of free will is much more important and relevant to autonomy and morality than the subatomic or metaphysical sort.
Much of the rest of the book is speculation about how this kind of free will might have evolved. This discussion was, for me at least, less compelling. Nevertheless, I found the book as a whole highly worthwhile.