The peacock is an example. The peacock's tail is long and heavy and actually diminishes its chances of survival. The tail needs a great deal of energy to grow and maintain, it reduces the bird’s agility, and increases the animal’s visibility to predators. Yet, it has evolved. This shows that peacocks with longer tails have some advantage.
So why did it evolutionarily survive? According to the principle, one day, a mutant peahen that likes peacocks with longer tail, looked for a male with this characteristic. The mature male she found was strong enough to survive even though it had a long tail and therefore it is probably stronger than the average peacock. Therefore, its predecessor males are going to be stronger and also to have a long tail while the newborn females are going to like long tails, passing the ideals of the principle to the next generations.