1Behold, how good and how pleasant it is For brethren to dwell together in unity! 2 It is like the precious oil upon the head, Running down on the beard, The beard of Aaron, running down on the edge of his garments. 3 It is like the dew of Hermon, descending upon the mountains of Zion; For there the Lord commanded the blessing-Life forevermore.
This Psalm is called a Psalm of ascension the reason being is that they would sing it while they were ascending to the Tabernacle that was on Mt Zion to worship the Lord. The Tabernacle that King David set up on Zion was a Tabernacle of continual worship, and worship is all that was allowed to take place there.
The message of the Psalm is that when brethren dwell together in unity it releases the priestly anointing oil to flow. When the anointing that only comes from unified worship