What does it mean to have a soft bone after surgery on the bone?

Osteopenia. It is not unusual to develop "disuese osteopenia" after fracture work. When you are not walking on a leg, the bones see less stress and lose some of their bone density. It should revert to normal with time.
See below. Often times during surgery the bone will be evaluated and soft bone is just that, soft bone, it may be in relation to osteoporosis.