The compiler for writing next generation JavaScript
You can find proposed ES syntaxes that currently can be used with Babel at its website https://babeljs.io/.
New ES syntaxes can be really powerful tools in our hands such as async-await instead of callback/promise hell, arrow functions, classes, decorators, destructuring, spread operator etc.
It can be included into your build pipeline. In browser based projects it can be combined with Webpack to be able to use not only new syntaxes but also CommonJs modules in the browser.