- Learn to say 'no' to the good so you can say 'yes' to the best. John C. Maxwell
Esperanto was created in the late 1870s and early 1880s by L. L. Zamenhof, a Polish-Jewish ophthalmologist from Białystok, then part of the Russian Empire. According to Zamenhof, he created the language to reduce the "time and labour we spend in learning foreign tongues" and to foster harmony between people from different countries: "Were there but an international language, all translations would be made into it alone (...) and all nations would be united in a common brotherhood." After more than a century since its inception, Esperanto is now spoken by hundreds of thousands - in fact, probably millions - of people all over the world. Today the objective of Esperanto is the same as always: to become the one foreign language everybody studies and learns.
While no foreign language is easy to master, Esperanto was intentionally designed to be as easy to learn as possible. Its grammar is concise and highly regular (no need to memorize verb conjugations), and its basic vocabulary consists of two thousand words or so - tens of thousands of words are formed with regular endings and other similar devices. You should be able to put Esperanto in use several times faster than any national language like French or Russian.
Esperanto may help you learn other languages faster
Strange as it may seem, if you are monolingual and invest a year in learning Esperanto and then four years in learning French, you may end up speaking better French than if you just learned French for five years straight (really, I am not making this up; there have been studies on children proving that).
How is that possible? Esperanto will teach you grammatical concepts (such as how to use various tenses, prefixes, endings, etc.) in a pure and easy-to-remember way. Grammatical concepts are always obscured by irregularities in natural languages, and it may take a lot of time to understand the same underlying principles without having any clear examples.