Mitosis

Cells in every living being need a new supply of new cells, as when you are growing, repairing or undergoing development you need new cells to replace the old cells, like our skin cells which will always fall off our bodies and need to replace themselves.

Every cell has a "Cell cycle", along the lines of.
Growth, DNA replication, Mitosis and Division(Cytokineses).

Firstly the cell grows in size, and now it has more Mitochondria and Ribosomes, usually when a cell is not trying to reproduce and duplicate its DNA is now a noodle-like substance, but once it is ready to duplicate the DNA condenses into Chromosomes.
These affect things in humans like eye colour, Eukaryotic cells like animal cells have two copies of chromosomes which are named 'pairs' of chromosomes, one chromosome is from their father while the other is from the mother.

Humans have 23 pairs of chromosomes so in total we have 46 chromosomes, but other species of life have different numbers of chromosomes, this is why animals cannot reproduce with other animals with different numbers of chromosomes like if an elephant and a bird were to have offspring they have different numbers of chromosomes so it would not work.

So when a chromosome wants to divide into more chromosomes, it first duplicates the 46 chromosomes, but the duplicate stays attached, so you get these X-shaped chromosomes, you might hear people talk about these chromosomes that have "arms". The right arm has the same amount of DNA as the left arm. Keep in mind this is all happening inside a human cell.

Once the cell is ready to divide, the chromosomes start to move to the centre of the cell, and after fibres or microtubules start slowly pulling the arms of both sides of the chromosomes splitting them in half and pulling them to each side of the cell. Pulling it to the 'poles' of the cell, and this breaks the cells apart, this happens to all 46 chromosomes inside the cell.

After all of that, we are finally able to do Cytokeneis or division, the cell membrane and cytoplasm pull apart making two daughter cells.
With the same DNA and the 46 chromosomes split between them, so each cell has 23 chromosomes, and then they repair, and undergo the same cycle infinitely repeating and duplicating.