A repair log was a log of repairs made.
In 2373, when Constable Odo suspected sabotage aboard Deep Space 9, he told Chief Miles O'Brien that he had checked repair logs hoping to match someone's movements to the locations where the alterations were made, but was unable to do so. O'Brien told him that they had a matrix system failure the day before, causing the files to be lost. Unbeknownst to Odo, it was actually O'Brien who had done the sabotage, working on the orders of a Pah-wraith that had possessed his wife. (DS9: "The Assignment")