'A plain' signifies the extremes of the Word regarding doctrine, and also the extremes of the church, or people in extremes of truths and goods. It also signifies good and truth in the natural self, because 'the people who dwell in plains,' or 'beneath mountains and hills' are in the extreme heaven. In Genesis 19:17, this signifies all doctrinal matters.

In Genesis 11:2, this signifies that their worship became more impure and profane. (Arcana Coelestia 1292)

In Jeremiah 21:13, this signifies the faith in which is no charity. (Arcana Coelestia 4715) Plain and valley signifies the ultimates of the Word. (Apocalypse Explained 411) Plain signifies good and truth in the natural man, because people in the First Heaven dwell in plain or valley. (Apocalypse Explained 449)

(References: Apocalypse Explained 400)

3772. 'And water the flock' means that the doctrine was drawn from it. This is clear from the meaning of 'watering', or giving a drink, as giving instruction, dealt with in 3069, 3768, and from the meaning of 'the flock' as people who are governed by the goods and truths of faith, dealt with in 343, 3767. Thus 'watering it' means giving instruction from the Word, and therefore from doctrine.