Metastasis describes what happens in a cancer patient when cancerous cells break off from a tumor or other main body, travel through the bloodstream, and start growing elsewhere in the body. Generally speaking, after metastasis occurs, the cancer cannot be easily treated and the patient may be terminal.
When in a central body, tumors can often be surgically removed or treated with targeted radiation therapy. However, once the cells travel elsewhere in the body, there is usually no way to target them individually. Surgery is pointless and radiation would have to be spread over the whole body, which would destroy bone marrow and the immune system, as well as resulting in radiation sickness.
One treatment of last resort is chemotherapy, which can destroy cancer cells everywhere in the body. However, chemotherapy has devastating side effects that can often be fatal by themselves.