Vestigial structures are those that used to serve a purpose, but through the process of evolution, no longer do. For example, an ostrich and an emu have wings, but they are no longer used for flying; the wings are vestigial. So are the human wisdom teeth.
One might think of vestigiality in matters beyond anatomy:
I know I'm stretching the boundaries of the concept, but I think the idea of vestigiality is useful when applied liberally: "In your life or business, what's something that used to serve a purpose but no longer does?"