A structure that is vestigial is one that has no or little function compared to the usage in the first and original ancestor. Vestigial organs were 100% needed for our ancestors to survive but now serve little purpose and might even be a bother. One example is our wisdom teeth. Our ancestors used wisdom teeth to chew food but over the past years we began to chew less. Because we chew less our jaws became smaller than our ape ancestors, not leaving room for our wisdom teeth. Wisdom teeth were mainly used to eat hard seeds and grains, now that humans mostly eat vegetables and meats, there is no need for wisdom teeth.