ECMAScript 6 introduces a new kind of property keys called symbols. Symbols are non-string keys with which are guaranteed to be unique and therefore allow properties to be added without the risk of conflicts with other keys.

Symbols are used in a lot of places in ES6 which makes them a good next feature to support.

They will be completely invisible to code running in ES5 mode. In ES6 mode, a Symbol global object is added to the global scope that allows creation of symbol values.