The Export tab of the Health Dashboard will display you any failures and expected failures this site faced or will face when exporting (for an explanation see below):
If any export attempt has failed, it will be summarized here. These mainly include Drupal Entity API failures and request failures. Check out the Entity Status page to see what entities failed exactly and optionally trigger their Export again.
When configuring a Flow there are several configuration options available for only selectively exporting entities. These include the "Ignore Unpublished Content" settings from nodes and media elements as well as the "Restrict to menu" setting from menu items. If an export has been ignored due to such a configuration, it will be summarized here. Usually you can just ignore all of these- in case you're wondering why an entity has not been exported without any error message, this is a good starting point. Check out the Entity Status page to see what entities were ignored exactly and optionally trigger their Export again.
If you add an auto-export Flow to an existing site where you already have content available, you will need to use the "Push All" action at the Flow to initially export all of the already existing content to the Sync Core. Otherwise these existing content items won't be available at other sites. If you have any of such entities, this will give you a summary of it. Just use the "Push All" action of the Flow to resolve this.
If you change the fields of an entity type, you need to re-export the content so that the changed field values are also available in the Sync Core and any other connected site. This will give you a summary of all entities that require another export.