Polaris Core consists of generic components that almost every web application needs.
If you take a look at it source, you will see something like this:
...
dist/
css/
fonts/
images/
js/
src/
scss/
ts/
...
dist directory contains all the files and libraries you need for production.
css directory contains files for Polaris CSS library.
js directory contains files for Polaris JavaScript library.
src directory contains all the source files you may need for development.
scss directory contains all Sass files for customizing Polaris CSS library.
ts directory contains all TypeScript files for customizing Polaris JavaScript library.