The definition below is from a government cancer website. But basically there are a few that my oncologist will check with periodic blood work. An increase in the number could suggest cancer cell activity or reproduction but could be caused by a lot of other inflammatory processes in the body. From what I understand, a decrease is never bad news.

"A tumor marker is anything present in or produced by cancer cells or other cells of the body in response to cancer or certain benign (noncancerous) conditions that provides information about a cancer, such as how aggressive it is, what kind of treatment it may respond to, or whether it is responding to treatment."