When she first appears, she is a small child from the 30th century who visits the past to seek help from the series' primary heroines, the Sailor Senshi. She later returns, a few years older, in order to train as a Senshi herself — Sailor Chibi Moon.
Note that her full name is Tsukino Usagi, the exact same as her mother's. She is given the nickname Chibi Usa in order to differentiate between the two Usagis. 'Chibi' means 'small', while 'usa' is the first part of her full name, so it means 'small Usagi'.