This is a process where sex cells (gametes) are created.
During this process the chromosomes in the original cells undergo special movements to ensure that the diploid number (in humans that is 46) is exactly halved to the haploid number.
That means that sperm cells and egg cells in humans have only 23 chromosomes. However they do not just get any 23 but a randomly selected combination of the 23 types of chromosome contained in the original cells. To simplify this let us imagine that there are only two types of chromosomes 1 and 2. Each cell contains a copy inherited from the father (paternal) and a copy inherited from the mother (maternal). These copies are not identical they are known as homologous. This means they have the same genes in the same places (loci) and same order but not the same alleles at each locus.