What is "export default" in JavaScript?

It's part of the ES6 module system, described here. There is a helpful example in that documentation, also:

If a module defines a default export:

// foo.js
export default function() { console.log("hello!") }

then you can import that default export by omitting the curly braces:

import foo from "foo";
foo(); // hello!

Update: As of June 2015, the module system is defined in §15.2 and the export syntax in particular is defined in §15.2.3 of the ECMAScript 2015 specification.

export default is used to export a single class, function or primitive from a script file.

The export can also be written as

export default function SafeString(string) {
  this.string = string;
}

SafeString.prototype.toString = function() {
  return "" + this.string;
};

This is used to import this function in another script file

Say in app.js, you can

import SafeString from './handlebars/safe-string';

A little about export

As the name says, it's used to export functions, objects, classes or expressions from script files or modules

Utiliites.js

export function cube(x) {
  return x * x * x;
}
export const foo = Math.PI + Math.SQRT2;

This can be imported and used as

App.js

import { cube, foo } from 'Utilities';
console.log(cube(3)); // 27
console.log(foo);    // 4.555806215962888

Or

import * as utilities from 'Utilities';
console.log(utilities.cube(3)); // 27
console.log(utilities.foo);    // 4.555806215962888

When export default is used, this is much simpler. Script files just exports one thing. cube.js

export default function cube(x) {
  return x * x * x;
};

and used as App.js

import Cube from 'cube';
console.log(Cube(3)); // 27