Once the genotypes of the parents in a cross have been determined, the next step is to determine the types of gametes each parent can produce.  If a parent is heterozygous, as is the case here, that means that one of the homologues contains the gene A and the other homologue of the pair contains the gene a.  As this pair of homologous  chromosomes moves to opposite poles in meiosis, half of the resulting gametes will contain the homologue with A and the other half will contain the homologue with a.  Remember gametes are haploid.  When this is translated into letters that stand for alleles, each gamete will have only ONE letter of each type.