My doctor says I have bone cancer, but I don't understand. What does it mean?

Metastatic cancer. Most patients will report "i have bone cancer" when their oncologist has told them their cancer has spread to the bone. This is most commonly a cancer they had previously that has metastized to the bone through the blood stream. Primary bone cancer is actually rare.
Specifics matter... Specifically, bone cancer is a type of sarcoma. It is more common in children/ young adults and usually one hopes to detect it early enough that surgery and / or radiation would be helpful. Other confused diagnoses with this term improperly used are cancers that start someplace else and spread to the bone (breast cancer for instance) or cancers of the bone marrow (leukemia).