Complex sentence: Heterozygosity refers to the state of being a heterozygote, but in population genetics, it commonly refers to the fraction of individuals in a population that are heterozygous for that locus.
Split into simpler sentences: Heterozygosity refers to the state of being a heterozygote. In population genetics, it is commonly extended to refer to the population as a whole, i.e. the fraction of individuals in a population that are heterozygous for that locus.