an untranslated Aramaic prayer to the risen Jesus, "Our Lord, come!"
Maranatha is not really a Greek word. It is a short Aramaic phrase that Paul left untranslated when he wrote to the Greek-speaking church at Corinth, because by then it was already a fixed cry used in worship by the earliest Aramaic-speaking believers. The original handwritten Greek had no spaces between words, so scribes read the single string of letters in more than one way, and that ambiguity is the whole story of the phrase.
The widely held reading today breaks it as "Our Lord, come!" (the word division behind most modern translations and printed Greek New Testaments). On this reading it is a petition, asking the Lord Jesus to come, the same plea given in plain Greek at the very end of the Bible: "Come, Lord Jesus" (Revelation 22:20). A long-standing alternative breaks the letters differently to read "Our Lord has come," turning the phrase from a request into a statement of faith.
Either way, the single most important thing the phrase shows is what its first users assumed: that the risen Jesus could be addressed directly as "Lord" in worship. The Aramaic word for "Lord" (מרא) here is the same one Jewish writers of the time used absolutely for God himself, so this brief prayer is one of the earliest signs that Jesus was honored as Lord within years of his death, in the language of his first followers.