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:
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
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.
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.