Hi itsyspider,
I think that your teacher was referring to adaptation for the purposes of mating. The most obvious example is the peacock's tail with its fantastic colour and reflective 'eye' patterns not present on a peahen. The plumage of a peacock is designed to attract peahens and nothing more. It does not allow the peacock to run faster or fly better or evade predators; i.e. it has no utility outside the important business of attracting a mate. In fact, there are costs associated with building such a plumage which the female does not pay. The bright plumage is attractive to prey and it takes energy and resources to grow and maintain it. However, although the tail must be expensive to build and maintain it obviously meets the criteria of fitness in terms of producing new peafowl so it is obviously a risk worth taking.