What does it mean to have calcium within a brain tumor?

Non-specific. Calcification is most commonly seen in benign brain tumors called meningiomas, and in low-grade malignant brain tumors such as oligodendrogliomas. However some high grade tumors can also have calcifications, so one can't rely on this too much to predict the behavior of a brain tumor.
Slow growth. As a general rule of thumb, calcium within a tumor suggests (but does not guarantee) slow growth.