Bone resorption is the gradual loss of bone. Bone is under a constant process of resorption and formation. As we age formation lessens and after a peak bone mass is achieved bone mass remains stable (resorption and formation are equal).
Osteoclasts are the principal cells responsible for bone resorption and an antiresorptive is something that works against bone loss due to osteoclastic activity.
When damaged areas of bone need to be replaced these bone-destroying osteoclasts dig tunnels or trenches by dissolving packets of old bone. As the old bone is broken down calcium is released into the bloodstream. The osteoclasts detach as another group of cells takes over to build the new bone.