When an organism's tiny building blocks undergo a process of splitting into two parts, that's cell division. Another name for cell division is "mitosis."
If you study biology, you'll learn about cell division, when a cell divides into two smaller "daughter cells." During cell division, all the tiny elements of the cell also divide — including the cell's chromosomes, nucleus, and mitochondria. The new cells are genetically identical to the original cell. In addition to mitosis, the simple division of one cell into two, there's also meiosis, when a cell splits into four new cells, each with half the original number of chromosomes.