Babel is a JavaScript compiler

Babel is a toolchain that is mainly used to convert ECMAScript 2015+ code into a backwards compatible version of JavaScript in current and older browsers or environments. Here are the main things Babel can do for you:

• Transform syntax
• Polyfill features that are missing in your target environment (through a third-party polyfill such as core-js)
• Source code transformations (codemods)
• And more! (check out these videos for inspiration)

// Babel Input: ES2015 arrow function
[1, 2, 3].map(n => n + 1);

// Babel Output: ES5 equivalent
[1, 2, 3].map(function(n) {
  return n + 1;
});

For an awesome tutorial on compilers, check out the-super-tiny-compiler, which also explains how Babel itself works on a high level.