1A farewell greeting — literary or old-fashioned, similar to 'farewell' or 'goodbye'.
さらば、また会う日まで。
Farewell, until the day we meet again.
古い手紙の最後に『さらば』と書かれていた。
The old letter ended with 'さらば'.
2A conjunction meaning 'if so' or 'in that case' — somewhat archaic.
さらば君の言う通りだ、計画を変更しよう。
If that's the case, you're right — let's change the plan.
彼が行かないのなら、さらばこちらも行かなくてよい。
If he isn't going, then in that case we don't have to go either.