Can someone tell me what this Psalm 133:3 mean? Thanks

Psalm 133
1: A song of Ascents; of David.
Behold, how good and how pleasant it is
For brothers to dwell together in unity!
2: It is like the precious oil upon the head.
Coming down upon the beard;
Even Aaron's beard,
That comes down the collar of his garments;
3: Like the dew of Hermon,
That comes down upon the mountains of Zion;
For there the Eternal commands the blessing,
Even life for ever.

The psalmist speaks of the blessed joy of brotherly concord. He compares it with the sacred oil with which Aaron was anointed and also with the dew that descends upon the mountains of Israel.

Hermon is the tallest mountain in Israel. The commentator, Kimchi, says that the word "dew" must be taken to refer to both sentences.

Dew is plentiful in Israel and is almost as important as rain. There are special prayers for rain said at the end of Sukkot, the beginning of the rainy season in Israel, and other special prayers for dew, said at the end of Passover, the beginning of the summer season.

The psalmist says that just as dew brings beauty and blessing, so it is when brothers live in harmony.

The psalm ends with the statement that God commands blessing there, in Zion. This is taken to mean the blessing of long life.