Sent: the total number of users estimated in the audience selection at time of send.
Delivered: the actual number of who received a push notification that was published in their Notifications tab.
Unsubscribed: the number of people that have opted out of push notifications since the last message was sent.
Failed: the number of people in the audience selection that the server encountered an error sending a push notification to.
Reasons a Push Notification Would "Fail"
The user uninstalled the app from their phone. (most common)
Issues with Apple or Google push notification servers. (uncommon)
General connectivity issues affecting the broader web. (uncommon)