Hematopoiesis is the process by which blood cells are produced. The document discusses the timeline of fetal hematopoiesis, where primitive hematopoietic cells first appear in the yolk sac at 7 weeks, then the liver, spleen, and thymus take over hematopoiesis temporarily before progenitor cells migrate from the AGM region to the liver at 10 weeks, making the liver the major hematopoietic organ, and hematopoiesis finally shifts from the fetal liver to the bone marrow at 16 weeks of gestation.