The liferay-npm-bundler is a bundler (like Webpack or Browserify ) that targets Liferay DXP as a platform and assumes you’re using your npm packages from portlets (as opposed to typical web applications).
The workflow for running npm packages inside portlets is slightly different from standard bundlers. Instead of bundling the JavaScript in a single file, you must link all packages together in the browser when the full web page is assembled. This lets portlets share common versions of modules instead of each one loading its own copy. The liferay-npm-bundler handles this for you.
This section of reference docs covers how Portal supports npm-based portlet projects with the liferay-npm-bundler.