In the Hardy-Weinberg equilibrium equation (p^2 + 2pq + q^2 = 1), the term 2pq represents the genotype frequency of heterozygotes (Aa) in a population in equilibrium. The term p^2 represents the frequency of dominant homozygotes (AA) and the term q^2 represents the frequency of recessive homozygotes (aa). p represents the allele frequency of allele A, and q represents the allele frequency of the allele a.
The sum of all of these terms is always equal to 1 (ie. p^2 + 2pq + q^2 = 1 ) because those three frequencies represent all possible combinations of alleles. This description works best for a simple case study where there is a single gene locus and only two alleles for that gene locus.
