Let's check the acronym first
ES = ECMAScript (ECMA - "European Computer Manufacturers Association")

ES is a standard for scripting languages. The most popular language among them is Javascript. It was first introduced in 1997.

When you hear people say ES5, ES6, etc, they are referring to a version of ECMAScript. ES6 is nothing but the standard released in 2015, i.e., ECMAScript 2015. There are many other standards of ES - ES7 (ECMAScript 2016), ES8 (ECMAScript 2017), etc.

Why is ES6 so important?

That is because it was the biggest update since its release. It is the 6th edition of the ECMAScript Language Specification and adds significant new syntax for writing complex applications. The subsequent updates are much smaller so the vast majority of the things you need to be familiar with are the things that were standardized in ES6.

Why should we even learn it?

ES6 has a lot of forward-thinking ideas attached to it and is a really exciting programming language to use. It is really fast and efficient. ES6 is 100% backward compatible. This means that when you start writing ES6, you can start with the regular JavaScript you know and love. You can then slowly start embracing new features and adopting the aspects of ES6 that make your life easier as a developer as you get more comfortable over time.

In Short:React Js recommends using the ES6 standard of Javascript.

Here goes some important ES6 features:
• let and const
• Arrow functions
• Destructuring
• The spread and rest operators
• Template literals
• Modular javascript using export/imports

Hope this info gives some idea about ES6 :)