Umbilical cord blood stem cells are powerful, unspecialized cells found in the blood left in the placenta and umbilical cord after birth, acting as a rich source of hematopoietic stem cells (HSCs) and mesenchymal stem cells (MSCs). These cells can self-renew and differentiate into various blood and tissue cells, making them crucial for rebuilding immune and blood systems in therapies for cancers, genetic disorders, and immune deficiencies, with cord blood offering advantages like higher maturity and lower immune rejection compared to adult sources.