Elm describes itself as a "delightful language for reliable webapps". It aims at producing web applications with great performances and no runtime exception.
Concretely, it is a functional programming language compiling to JavaScript. Its syntax and features originated from the ML family of languages, such as Haskell, but with a strong emphasis on learnability and simplicity.
You can learn more about elm on the official website, elm-lang.org. Newcomers should start with the elm guide, located at guide.elm-lang.org.