Psalm 133 is part of a series of 15 psalms called the "Songs of Ascent."  We're not quite sure what that means, but the best theory is probably that these psamls were intended to be sung by pilgrims as they "went up" to Jerusalem.  Scholars also point out that there were 15 steps between the courtyard of the Gentiles and the courtyard of the Jews, where the festival sacrifices were made.

In either case, these psalms were seen as special preparation for special acts of worship.  Each of the psalms is rather short, usually encompassing a single poetic thought.

Psalm 133 is clearly a psalm about unity.  We are told that living together in fellowship is like "precious oil poured down on the head," or like "Dew on Mount Hermon."  Both of these images are images of refreshing coolness in the desert heat.

The oil image, however, carries a double weight.  Oil poured on the head of Aaron would have been a priestly anointing, a sort of ordination.  The symbolism of the pslam is reminding us that our fellowship with one another is not merely a refreshing presence in the midst of a  difficult world, but also acts as an emblem of authority.

Because of our love for one another, we are ordained as God's priests in the world.