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)

9936. 'And it shall be on Aaron's forehead' means from the Lord's Divine Love. This is clear from the representation of 'Aaron' as the Lord in respect of Divine Good, which is the Good of His Divine Love, dealt with in 9806; and from the meaning of 'forehead', when the Lord is the subject, as His Divine Love. 'The Lord's face', which is the same as 'Jehovah's face', means all attributes of Divine Love, such as mercy, peace, goodness, or wisdom, 222, 223, 5585, 6848, 6849, 9306, 9545, 9546. These are meant by 'Jehovah's face' or 'the Lord's face' because in general 'the face' means a person's interiors, that is, a person's affections and consequent thoughts, thus the things which constitute his love and faith, see the places referred to in 9546. These are meant by 'the face' because they shine from the face, as if seen in their imprint or image, which also is why the face is called the image of the mind. So it is that when 'face' is mentioned in connection with Jehovah or the Lord the attributes of His Divine Love are meant. 'Forehead' in particular means Divine Love itself, because interiors have been allotted their own provinces in the face. The interiors that belong to love reside in the province of the forehead, those belonging to wisdom and intelligence in the province of the eyes, those belonging to perception in the province of the nose, and those belonging to utterance in the province of the mouth.

[2] From all this it is evident why 'forehead' - when the Lord, represented by Aaron, is the subject - means Divine Love. Since someone's forehead corresponds to his love those governed by celestial love, that is, by love to the Lord derived from the Lord, are said 'to have a sign on their foreheads', meaning that they are under the Lord's protection because they abide in His Love, as in Ezekiel,

Jehovah said, Go through the middle of Jerusalem and make a sign on the foreheads of the men (vir) who groan and sigh over all the abominations done in the midst of it, and strike; do not let your eye spare. But against any man (vir) on whom there is a sign do not go near. Ezekiel 9:4-6.

In John, in the Book of Revelation,

Behold, a Lamb standing on Mount Zion, and with Him a hundred and forty-four thousand, having His Father's name written on their foreheads. Revelation 14:1.

In the same book,

They will see the face of God and of the Lamb, and His name will be on their foreheads. Revelation 22:4.

In the same book,

It was declared that they should not harm the grass of the earth, or any green thing, or any tree, but only the people who did not have God's sign on their foreheads. Revelation 9:4.

[3] 'Having God's sign on their foreheads' and 'having God and the Lamb's name on them' stand for their being kept safe from molestation by evils from hell, because they abide in the Lord through love. 'The grass' and 'any green thing', which were not to be harmed, stand for true factual knowledge by means of which the truth of faith develops, 7571, 7691; and 'any tree', which too was not to be harmed, stands for the perception of truth springing from good, 103, 2163, 2722, 2972, 4552, 7692.

[4] In Moses,

You shall love Jehovah your God with all your heart, and with all your soul, and with all your strength. You shall bind these words as a sign onto your hand, and let them be as frontlets between your eyes. Deuteronomy 6:4-5, 8.

'As frontlets' stands for as a sign of love to Jehovah God. The words 'between your eyes' are used because intelligence and wisdom which are born from that love are meant by 'eyes'; and wisdom born from that love consists in having God constantly before one's eyes. This is self-evidently so because the subject is love to Jehovah God. The declaration that they should love Him with all their heart, with all their soul, and with all their strength, means that they should do so with all their human powers. 'With the heart' means with the will where the good of love resides, 7542, 9050, 9300, 9495; and 'with the soul' means with the understanding where the truth of faith resides, and so with faith, 9050. These two powers belong to the internal man. 'With all their strength' means with those powers of will and understanding as they exist in the external man. The strength and power of the love of both, of the external man and of the internal, are meant by 'hands', 4931-4937, 7518; and this is why it says that those words were to be bound 'as a sign on the hand'.

[5] Since 'the forehead' by virtue of its correspondence means celestial or heavenly love with those who are good, with those who are bad it accordingly means hellish love, which is the opposite of heavenly. The forehead of the latter is called a bronze forehead in Isaiah 48:4, and an obstinate forehead in Ezekiel 3:7-8. And in reference to those ruled by hellish love it is said that they had the mark of the beast on their foreheads, Revelation 13:16; 14:9; 20:4, and also the name of Babylon on their foreheads, Revelation 17:5.