A stem cell is essentially a “blank” cell, capable of becoming another more differentiated cell type in the body.
Microscopic in size, stem cells are big news in medical and science circles because they can be used to replace or even heal damaged tissues and cells in the body. They can serve as a built-in repair system for the human body, replenishing other cells as long as a person is still alive.
Since they have not yet specialized, stem cells can respond to different signals and needs in the body by becoming any of the various cell types needed, e.g., after an injury to repair an organ. In that sense, they are a bit like a maintenance crew that keeps repairing and replacing damaged or worn-out cells in the body.