Which portion you don't get?
Selective breeding means some important genes are going to vanish. Suppose you put a white eyed and a red eyed male and a white eyed and a red eyed female Drosophila in a container. After fifteen generations, there are red eyed individuals in abundance, but the white eyed are less. This is because both the red eyed female and the white eyed female prefer to mate with the red eyed male. And the white eyed male, left alone, ultimately produces little progeny. So alleles for white eyed trait ultimately become less, and the allele tends to get fixed.(If all individuals in population are homozygous for a trait, its alleles become fixed and no variability is produced.)
It's confusing, yea, but I think It might be helpful.