Meiosis
The two chromosomes in a pair are not identical because at any particular locus on the chromosomes there may be different alleles (forms of the gene) on the two chromosomes. Think about each pair of chromosomes, one set from each parent. From each pair, only one goes into each gamete. So, in the first place, gametes differ because of each pair the gamete may have the father's chromosome or the mother's. This is called 'independent assortment'.