The Hardy-Weinberg equilibrium model tries to explain why genotype frequencies may remain constant within a population. Hardy-Weinberg equilibrium in its simplest form is based on the existence of two alleles at a single genetic locus. These are generally called p and q. If we make the assumption that the population is large (effectively infinite), mating is random and that there is no difference in fitness between the genotypes, given sufficient time the genotype frequencies will be:

Genotype      pp     pq      qq
Frequency     p2     2pq     q2

This should (assuming the above assumptions are true) be stable - although the genotype frequencies may be different to the starting point, the allele frequencies should be the same as before. Equilibrium has therefore been reached. Sadly for the Hardy-Weinberg equilibrium model, the above assumptions are rarely true. In most cases, genetic equilibrium is reached by strong selection pressure in opposite directions stabalising the population.