RECORDS EXCLUDED FROM SYNC lists records that are not supposed to be in sync, either because they do not match any of the filters that have been setup inside of the sync, or because they don't have an email address. In the latter case, they can be included in the sync by selecting the 'Avoid duplicates' option of a contact sync.
FAILING TO SYNC lists contacts no longer kept in sync. There are many reasons for this: a required field is missing, fields contents cannot be synced, HubSpot detected a duplicate, etc. The reason why the sync failed will be displayed in the tooltip. The information is fed directly to HubSpot from the third party app and navigating to the tooltip will allow you to fix the error so the sync can work properly.