Muscle is built through a process in which tiny tears in our muscles are repaired even stronger during recovery. Your bones are similar.
When you perform resistance exercises, there is a healthy increase in the pressure or "stress" that is being put on your bones. This increased stress prompts your body to respond by releasing chemicals that kickstart specialized cells to help with bone repair and growth. These new cells eventually turn into bone cells that add thickness and strength to your existing bone material. Your body will respond to the increased load by becoming stronger, and more able to tolerate the load in the future. The whole process results in new bone material that contributes to stronger bones.