Calcium is a dietary mineral found in dark green vegetables, cheese, yogurt, and milk. Certain grains, legumes, and nuts are also good sources of calcium. It is currently being studied as a way to prevent cancer.
One article (1) looked at calcium and the prevention of cancer, particularly colon cancer. It noted that, while it seems that eating calcium in the diet does help prevent colon cancer, the mechanism of action as to how it does this is not clear.
They also noted that calcium binds with bile acids in the GI tract, forming a soap that decreases the damaging effects that some foods have on the lining of the intestines. By doing so, calcium may actually prevent colorectal cancer if eaten in the food or used as a supplement.
Source: 1.Calcium and cancer prevention. http://www.cancer.gov/about-cancer/causes-prevention/risk/diet/calcium-fact-sheet. Accessed 9/11/16.