Aaron was the brother of Moses. He symbolizes two things, at different stages of the story.

During the first part of the exodus, when he was Moses' spokesperson, Moses represents the Word as it truly is, as it is understood in heaven, while Aaron represents the Word in its external sense, as it is understood by people in the world. This is why Aaron talks for Moses, and the Lord says of him "he shall be as a mouth for you, and you shall be to him as God." (Exodus 4:16)

Later, after the Tabernacle was built and he was inaugurated as high priest (see Leviticus 8,9), Aaron represents the Lord as to the Divine Good, and Moses represents the Lord as to the Divine Truth.

In Exodus 28:1, Aaron signifies the conjunction of Divine Good with Divine Truth in the Divine Human of the Lord. (Arcana Coelestia 9806, 9936)

In Exodus 32:1, Aaron represents the external of the Word, of the church, and of worship, separate from the internal. (Arcana Coelestia 10397)

In Exodus 4:14, before he was initiated into the priesthood, Aaron represents the doctrine of good and truth. (Arcana Coelestia 6998)

7295. 'And Aaron threw down his rod in front of Pharaoh and his servants, and it was made into a water-serpent' means that sheer illusions and resulting falsities reigned among them. This is clear from the meaning of 'throwing down the rod' as demonstrating power, dealt with just above in 7292; from the representation of 'Pharaoh and his servants' as those who molest by means of falsities; and from the meaning of 'a water-serpent' as illusions and resulting falsities, dealt with just above in 7293.

[2] This wonder means the first warning given to those who molest to leave off. The situation with the evil who molest the upright in the next life is this: When they first enter that life from the world they have good spirits and angels attached to them, as they did when they were people living in the body; for even wicked people also have angels present with them, the reason being that they are then able to turn, if they will, towards heaven, receive what flows in from there, and be reformed. Consequently, since their life in its entirety follows them into the next life, they live initially in association with angels. But as they are by nature such, because of the life they led in the world, that they cannot receive the inflow of truth and goodness from heaven, the angels and good spirits then gradually depart from them. And as these depart, those people become less and less rational. For the ability to be rational comes from the Lord by way of heaven.

[3] The first stage in the removal and deprivation of the inflow of truth and goodness is what is described here by Aaron's rod being turned into a serpent, meaning that sheer illusions would reign, and falsities resulting from them. The second stage is described by the waters of Egypt being turned into blood, meaning that actual truths were falsified. The third stage is that of the frogs which crawled out of the waters, meaning reasonings based on utter falsities, and so on. By such stages too the evil in the next life are deprived of their understanding of what is true and good.