TL;DR

Babel allows new features of JavaScript to work with old browsers.

Explanation

According to the official document, 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.

For example, if a developer uses an arrow function, Babel transforms the arrow function to its old format so that the arrow function can be compatible with web browsers

// ES6
[1,2,3].map((n) => n+1);

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