ES5 should be used in apps which require compatibility with older browsers or libraries.
ECMAScript 6 is the newer version, and is a superset of the ES5 specification. Code written in ES6 is generally less verbose than its equivalent code in ES5, unfortunately, it only works in the most recent versions of browsers.
This does not mean, however, that a developer needs to work with ES5 when targeting older browsers. There are several ES6 to ES5 transpilers, which convert the ES6 code to its ES5 equivalent automatically, and are designed to be integrated into front end web development toolchains.