Esperanto is an auxlang (short for auxiliary language). An auxlang is a language designed to be a common language between two people who would otherwise not share a language. Esperanto is the world’s largest auxlang, and also the world’s largest conlang (constructed language, which is the group auxlangs fall into, being a language constructed deliberately as opposed to a natural language which developed over time in the real word).
Zamenhof, the creator of Esperanto, was one of the first of a long line of future conlangers. He published his Esperanto grammar in 1887. Conlanging has grown as a hobby over the decades, but it has really flourished recently thanks to films and tv shows like Star Trek, Game of Thrones and Defiance which all feature constructed languages like Klingon, Dothraki, High Valyrian and Castithan.