The practice of saying “Amen” at the end of a prayer originated in ancient Jewish custom when most formal prayers or blessings were said by priests and the congregation would respond with agreement. Later “Amen” became the standard response to any prayer that began with the phrase, “Blessed are You, Lord our God…”
It is only used in response to the spoken prayer of another person, and is used in the sense of repeating the blessing. This single word has been the subject of much commentary in rabbinic literature, and still constitutes an important part of the liturgy and corporate worship in synagogues around the world.