A bone island, also called an enostosis, is a common, benign bone lesion which is usually seen as an incidental finding. It constitutes a small focus of cortical bone (compact bone) within the cancellous bone (spongy bone).
It is just a small area, where the body has produced a little bit of hard cortical bone, within the spongy cancellous bone.
True bone islands do not cause any symptoms. They are found incidentally when an x-ray is taken for some other reason.
If there is reason it believe that the enostosis is causing problems, then it needs evaluation, as some other process is going on.