Complex sentence: Purging occurs because many deleterious alleles only express all their harmful effect when homozygous so that, after inbreeding, they occur in homozygous individuals that are less fit and pass fewer copies of their genes to future generations.
Split into simpler sentences: Purging occurs because many deleterious alleles only express all their harmful effect in homozygosis. Therefore, after inbreeding, they occur in homozygous individuals that are less fit and that pass fewer copies of their genes to future generations.