CoffeeScript is a language which transpiles into JavaScript, used for making dynamic websites (both in the web browser, and on the server). The transpiler is used to transform CoffeeScript code into JavaScript before running, to keep compability and the same performance of JavaScript while providing many new features!

CoffeeScript was created to provide a simplified way to write JavaScript, based on a blend of the Ruby and Python languages. One difference is the use of indentation and arrows to replace JavaScript's verbose function code, and introduces useful features from functional languages including pattern matching.

CoffeeScript is one of many new languages that transpiles to JavaScript, including TypeScript and Flow - the community is  growing, and these types of languages are very popular.