The background processing feature allows you to process the carriers in the background, using WP Cron feature. This means while doing an action with should trigger the notification, instead waiting for the process to finish (and ie. send 100k emails) the action is loaded in the cron, and executed in the next cron call.
It's particularly helpful to distribute the server load but it also resolves the Gutenberg issue, where wrong custom fields or taxonomies were included in the notification.
To enable this feature navigate to Notification -> Settings and in Advanced section, check the Background processing checkbox. From now on, the actions will be executed via cron.