When a callout is performed and the percentage of the recipients are not met that callout is considered failed. You can select the retry failed option to resend the notification to the destinations, omitting the successful calls. This will resend the message to everyone who had a call report a Failed State.
In the screenshot below the original callout was performed and the notification feed had 5 users subscribed. On the first attempt, only 1 user acknowledged the call.
Clicking on Retry Failed resulting in the notification going back out to the feed but to only the 4 users who did not acknowledge the original alert. Thus resulting in the callout being placed to only 4 users. 1 user acknowledged the retry.
Clicking on Retry Failed again the message was only sent to 3 users this time as the other two members had acknowledged the alert from the 2 pervious attempts.