"Equilibrium is an important concept in population genetics. I think it is easiest to illustrate it by thinking of two genes each with two alleles. If there is nothing particular going on in the population then what we would expect is the association between the two alleles, both within each locus, and between the loci to be random. They will not be any more likely to co occur than you would expect on the basis of their separate frequencies in the population - that is known as linkage equilibrium. The opposite is linkage disequilibrium where there is a non-random association between the two alleles at different loci."