What does this equation mean?

One of the most widely used field-deployable methods for estimating $N_e$ from a single sample of genotypes is the linkage disequilibrium (LD) method , implemented and validated by Waples and Do in their 2008 paper introducing the LDNE software [ 3 ] . The logic: in an infinite population at linkage equilibrium, alleles at different, unlinked loci are statistically independent — knowing an individual’s genotype at one locus tells you nothing about its genotype at another. In a finite population, genetic drift generates chance nonrandom associations between alleles at different loci purely by sampling — associations that decay each generation but are continuously regenerated by drift in a population small enough for drift to matter. The magnitude of that drift-generated linkage disequilibrium, measured across many locus pairs, is therefore a signature of $N_e$ itself, estimable from genotypes drawn in a single sampling event — no need to sample the same population twice across time, which is the main practical advantage over temporal methods.