Complex sentence: During inbreeding, as related individuals mate, they produce offspring that are more likely to be homozygous, so that these deleterious alleles appear more often in homozygous individuals that are less fit and that pass fewer copies of their genes to future generations.
Split into simpler sentences: During inbreeding, as related individuals mate, they produce offspring that are more likely to be homozygous. Therefore, after inbreeding, these alleles appear more often in homozygous individuals that are less fit and that pass fewer copies of their genes to future generations.