Perhaps you heard about it in the 70's or you are much younger and you saw it on Duolingo.
Esperanto is a constructed language that was invented by an opthalmologist Ludwik Lejzer Zamenhof. His language project began in the 1870's and 80s. The first instruction book was published in 1887. The language took off quickly and many were inspired to learn it to foster international communication. Unfortunately it was considered a great language for spies and was quickly shot down by Hitler. But Esperanto survies!