In ES6, JavaScript provides language-level support for modules. It uses export and import to organize modules and create a static module structure. That means you can determine imports and exports at compile time. Another important feature of ES6's module is that imports and exports must be at the top level. You cannot nest them inside blocks such as if and try/catch.
Besides static declarations of imports and exports, there is a proposal to use the import() operator to programmatically load modules. The proposal is, at the time of writing, at stage 3 of the TC39 process. You can checkout the details at https://github.com/tc39/proposal-dynamic-import.
To create a module, all you need to do is to put your JavaScript code into a .js file. You can choose to use tools such as Babel (http://babeljs.io) to compile ES6 code into ES5, together with tools such as webpack (https://webpack.js.org) to bundle the code together. Or, another way to use the module files is to use <script type="module"> to load them into browsers.