We are built from cells that form tissues and organs. The free-radical theory of aging says accumulation of cell damage from oxygen (ROS) generated in metabolism eventually ends the balancing act of life. Crudely, we rust faster than we repair.
Biological longevity is sometimes seen as a ballance between cells that die too fast (senescence) and cells that reproduce too fast (cancer), but both are part of the same problem: the idea of antagonistic pleiotropy says that we animals have a hard time making structures which are effective in youth for growth and reproduction which also can handle cumulative (oxidative) stress of aging.
Increased cell reproduction (hyperplasia) of healthy cells is a normal stress response, but pushed too far it can becoming neoplasia, excessive cell growth (cancer). Under too much stress, especially among vertebrates if cells are not fairing well after prolonged inflammation, some can go on strike and stop playing a helpful role in a tissue. If cell demands for safety and sustenance are not met, some cells may start rioting, fending for themselves, reproducing too much, then breaking out and roving around the body in search of a better situation.
These metastatic cancer cells can be killed with various therapies, but new research shows that they might also be coaxed back into a calm state.