Znc-backlog is a third party ZNC module that makes it easy to request backlog. Its intended use is for when you have just launched your IRC client and gotten a few lines of backlog sent to you, but want to read more. Instead of having to deal with shelling into the box where you run ZNC and manually sifting through the logs, you can issue a short command in your IRC client to request any amount of the most recent lines of log.

For more information about ZNC, see the znc package.