To expunge is to cross out or eliminate. After Nicholas proved he had been in school on the day in question, the absence was expunged from his record.
Expunge is often something you do to a document. When government censors block out text in documents before making them public, they are expunging the text. You can also use the word in a more metaphorical sense. The principal tried to expunge all traces of bullying from the school by implementing a kindness initiative and treating all complaints as serious.