A Punnett square shows the possible phenotype and genotype of both parents for the offspring because it lists the possibilities of a certain trait that the parents can pass the the offspring. In other words let's say mom has brown hair (BB) and dad has blonde hair (bb.) If we do a Punnett square we can list the possibilities of the offspring either having brown or blonde hair. In this case since brown hair is dominant to blonde there is no way if mom is homozygous (two Big B's) that the offspring can have blonde hair.
-Sorry if it's confusing I hope I helped a little!
