It represents the genotype.
For example, Pp is heterozygous genotype, pp is recessive homozygous genotype, PP is dominant homozygous genotype. But, combination and dominance between alleles can show different phenotypes. For example, complete dominance is when phenotype of the dominant allele is expressed in heterozygous. Codominance is when both alleles are expressed equally in heterozygous (phenotype is a combination of both).
