What is an ES6?

• ECMAScript 6 (ES6) is the sixth major release of the ECMAScript language specification. ECMAScript is the “proper” name for the language commonly referred to as JavaScript. 1

Where can I use this new syntax? Is it supported?

• You can see which browsers support which features here, in the compatibility table.
• You can also transpile your code from ES6 to the more widely supported ES5 we all know and love. Lots of folks use Babel.

Why should I learn it?

• Awesome new features let you bind context (this) more easily, write more concisely (no more writing function a hundred million times), handle ansynchronicity (I Promise hyuk hyuk) more easily, and communicate your intentions to other developers (let vs. const for example). Check out this article for some thoughts!
• You'll probably see it a lot now if you write any React code, but you'll be seeing a lot of these features in more places as time goes by.