Esperanto is a constructed language, conceived to be easy to learn and to understand, and intended to be used as the international exchange language between persons of different native languages.

Rather than trying to learn the other national languages, which would require a considerable effort for not always a quite encourraging result, the idea is to learn a common language and thus making one step towards the other for being at equal level in communication.

Obviously, the aim is not to replace the national languages, but to be a second language common to everyone.

Esperanto : the fair language

For further information : article from Wikipedia about Esperanto