CSS aggregation bundles multiple files so they can be downloaded more efficiently.
Enable it at Configuration > Development > Performance.
Aggregated files will have hashed names.
When a file is changed, the assets need to be regenerated by clearing the cache.
Disable aggregation for local development, see web/sites/example.settings.local.php.
How it works
Drupal creates 2 CSS bundles for the header. One contains global styles and the second is dependencies on the page.
The assets that have been sent to the user are contained in a session variable so they can be excluded from subsequent bundles.