An amanuensis is a secretary who transcribes what is spoken by the "author" to produce an original copy of a manuscript, known as the "autograph".
The autograph manuscript would most likely be written in two handwritings, the majority by the amanuensis and a closing message by the author. The author's signature would mark that the dictated text and any self-edits were genuine.
The resulting manuscript may be in the author's language or translated into another language.
The most well-known New Testament amanuensis who translated the author's words is John Mark, also known as Mark the Evangelist. Simon Peter, who went on to start the Catholic Church in Rome, spoke Aramaic, Latin, and possibly Hebrew, but he could not speak Hellenistic Greek, so John Mark translated his words into what became the Gospel of Mark, most likely the earliest of the synoptic gospels.
Paul had several amanuenses who assisted him in writing and transporting his epistles (letters) to the early churches along the Mediterranean Sea, when he was imprisoned in Rome. In some cases, he would mention these men in the epistles themselves. When he did not, textual critics identify the use of an amanuenses by the shift in tone of the text toward the end.