It's important to know that your teeth are suspended inside your gum and bones. They are not fused into your bone. The teeth are surrounded by the periodontal ligament (PDL). This is what enables your teeth to be moved. Braces use gentle force in one direction. This force compresses the PDL on one side and stretches it on the other. On the side that is compressed, the bone reabsorbs. On the side that is being compressed, the bone grows. So actually, your teeth move as a product of simultaneous bone loss and creation. This can seem complicated and confusing. Actually it's a very simple natural phenomenon that is well explained in this video.