The Repair feature allows you to manually replay all Transactions on an Integration which are in either an Error or Timed Out state.
Repair is a UI Action on the Integration. Clicking it will cause a bulk replay of all its broken Transactions (those in an Error or Timed Out state).
You can repair an Integration either from Unifi Integration Designer, or native ServiceNow:
Unifi Integration Designer
Native ServiceNow
Click the Repair UI action on the Integration record in the Unifi Integration Designer portal.
Click the the Repair UI Action in the header of the Integration record in native ServiceNow
Unifi Admins can repair Integrations.

Use Case

In the case of a major outage (where perhaps the other system is down, or the authentication user credentials have been updated), you might have a number of failed Transactions. Rather than stepping into each Transaction and replaying them individually, you can simply replay all the broken Transactions on the Integration. This could represent a significant time-saving.
As a further aid, there is also a 'View broken transactions' Related Link on the Integration record which will take you to a list of all Transactions in an Error or Timed Out state.
Hints & Tips: It is possible for Unifi to be proactive and raise Incidents about Integrations (see Event Actions). In such a case, the repair functionality could be tied to the resolution of the Incident, whereby Unifi automatically runs 'Repair' on the Integration the Incident was raised against.

Replay, Repair and Ignored

When a Transaction is replayed (whether individually, or in bulk), the original record is set to 'Ignored' and a new Transaction (with a decimal suffix) is generated (taking the same Stage data & reprocessing it) and sent.