My 2c about what this is referring to:

In "the good old days", you would just import a script using a normal <script> tag, it would do stuff on the page, and you're done.

What a lot of people refer to as "modern" web front-end development, there is a whole toolchain needed to compile and prepare your site for deployment: babel, webpack, etc. While these front-end tools solve a host of problems, they also introduce a whole set of new ones, mainly around complexity and the fact that what is actually going on "under the covers" is a lot more opaque to the front-end developer.