Whenever there is a tiny crack inside the bone, the bone cells will dissolve the area around the crack and fill it in with new bone. At any one time this is happening in millions of places throughout the skeleton. This is called bone "remodeling".
The following movie, courtesy of Susan Ott, MD, Associate Professor of Medicine at the University of Washington, shows what happens when the bone breaks in one place. You'll see a little crack, then cells will turn into osteoclasts which dissolve (resorb) the bone, then cells from the marrow space will turn into osteoblasts (cells that form new bone) and build new bone.