Your body is made up of billions of cells, which are topo small to see without a string microscope.
Inside most of those cells are chromosomes, which are thread like strands that contain hundreds of genes that determine your physical traits.
Most people have 23 pairs of chromosomes, and you received half your chromosomes from mother, and the other half from your father.
Even after you are born, your 46 chromosomes continue to guide the way your body grows and develops.
The number of chromosomes is not related to intelligence or comlexity of the creature. Peas only have 12 chromosomes, while a dog has 78, and there is a species of caryfish with 200 chromosomes. There are even organisms of the same species with different numbers of chromosomes.