A postanal tail is an extension of the spinal chord that extends beyond the animal's anus. Postanal tails are a feature of all chordates, which is a phylum that includes vertebrates. All chordates have a postanal tail at some point, but they may not have that tail for their entire lives.
As vertebrates, humans are part of the chordata phylum, but most adult humans don't have postanal tails. This is because humans are among the vertebrates, including animals such as frogs, that tend to lose their postanal tails by the time they reach adulthood. Human fetuses have this feature, but the tails shrink as they develop and are typically gone by birth.
In technical terms, the postanal tail is an extension of the notochord and nerve cord that extend beyond the anus. The word postanal means behind or after the anus. This is any part of the body that extends beyond the anus; in many cases, a chordate's anus isn't at the posterior tip of the body. A dog's tail is considered a postanal tail, for example. Dog owners will be familiar with the tail's placement above and beyond the animal's anus. The postanal tail is one of several traits that all chordates have at some point in their lives, including pharyngeal slits.