This is a little out of date now, and some of the predictions are almost adorably wrong at this point — that we would understand dark matter and dark energy, and that we’d have a unified Theory of Everything explaining how all the forces we know of are tied together. But this book is still useful in explaining, in clear and simple terms, why exactly people say the universe has been “fine-tuned”. It’s not the most in-depth treatment out there, but I think it’d be very good for getting to grips with the basics.
In summary: there are several numbers underlying the universe which are constant, and they are very precisely definable down to multiple decimal places… and if you change them in any way, you make our existence as we know it impossible. There are problems with this, of course; life doesn’t have to look exactly like us to be viable, and of course we’re in a world that is perfectly tuned for us to exist. That doesn’t, in and of itself, prove anything. I know people often use it to support the idea of multiple universes, all varying slightly — but something can be made just once and be utterly unique and turn out to be perfect for something, even if you don’t make multiples.
This is, honestly, why I find physics so frightening. It’s all so terribly unlikely, and we don’t understand it, and against all this it becomes very apparent, to my mind, how small and alone and temporary each human being is.
It’s also fascinating, even for those who prefer biology as a science, like one you could probably name…