This will start an HTTP server at `http://localhost:4000/` that serves the docs built in the `_site` directory; and anytime the docs are rebuilt by you, it will serve the docs site on the fly. You can also add the `--open-url` option (or its alias `--o`) to automatically open the server URL in your default browser when it's ready.
In your main shell session where you develop, if you change anything in `docs/` the jekyll server will rebuild those on the fly. But if you change anything about the Bulma SASS or CSS, you need to do `npm run start` to build the docs' CSS before you will see it in the browser. The process running `jekyll serve` will pick up the new CSS automatically.