This allows you to delete imported transactions. Once deleted, you can import transactions for these days again.
You do this when you have imported a file from your banking software which contains transactions for part of a day. Further transactions for that day will not be included in any subsequent files you import. You must delete all the transactions imported for the partially reconciled day. Then you must import and match the transactions again.
Open: Cash Book > Period End Routines > Bank Reconciliation.