Browsers don’t understand ES2015 modules (if they have support it works a bit differently) so you need a bundler to transpile the code to something the browser can process.
Have a look at
or possibly this to see if you can get webpack 4 to work in a practice project (first).
In your journeys you may run into <script type=module></script> like in ECMAScript modules in browsers - don’t get distracted, learn what role bundlers play and how they work - when it comes down to it: resolve as much as possible during compile/build time and don’t put it off to runtime unless absolutely necessary.