This is a small, leftover form of something such as a body part or behavior that doesn’t have any function. If it’s a body part or organ, it might be much smaller than a functioning version in another species. Or it might only be partially developed.
For example, many mammals can swivel their ears, pointing them toward certain sounds. Humans can’t do that. Instead, we turn out heads. But we still have some tiny vestigial muscles around our ears. Those muscles are the leftovers of muscles our ancestors used to turn their ears. The only function those muscles serve now is as a party trick — when someone shows off by wiggling their ears.