Wolff's Law is a theory, developed by a German anatomist with the name of Julius Wolff (hence the name). The theory states that a healthy person's or animal's bone would adapt to the force it receives. In other words, the bigger the load that the bone receives, the bone will change/modify it's structure to adapt to the kind of load it is place under. This also applies if the load decreases. Your bones will too adapt to the environment and model the bone under the condition it is on.
An example of this are astronauts who are in zero-gravity for extended periods of time. Upon returning to earth, they will be extremely weak due to the conditioning of their bones and muscles to adapt the environment. Zero gravity exerts no load to the body, therefore the muscles and bones grow weaker.
On the other hand, martial artists condition themselves by repeatedly hitting objects. Many know it works but do not know why. It's just Wolff's Law. This does not happen overnight but possibly years or more depending on the intensity of the training. Muay Thai fighters took this very seriously and train their hands and legs, esspecially until they become deadly weapons. With rock solid bones that are impervious to pain, they are certainly opponents to be feared.
However an important factor to consider is if you hit an object that is harder than your bones, like a tree or a rock wall, you risk injury.
You don't want to end up like this guy.
An easy way to condition your bones is to hit boxing pads, that are hard enough. Do this a lot and you would definetly see an improvement. Another way, which I learned from Shaolin monks in China is that they suspend a phone directory, a thick one, on the wall, and punch on it. The book is soft, yet hard, and it makes a good buffer between your knuckles and the wall.
And none of this is easy. To train to such a degree is pure sweat and blood.