Psalm 132 is the fourteenth, or second last, of the gradual psalms (the last of the set concludes Compline each day in the Benedictine Office). At three verses as it is arranged in most bibles, it is one of the three shortest in the psalter.
The psalm points us to the unity that can only come through Christ, and so it is perhaps significant that it it is the fourteenth psalm of the group, a number associated with the coming of Christ by virtue of the three groups of fourteen generations listed in Matthew 1.
Canticum graduum David.
A gradual canticle of David.
Ecce quam bonum, et quam jucúndum * habitáre fratres in unum.
Behold how good and how pleasant it is for brethren to dwell together in unity:
2 Sicut unguéntum in cápite, * quod descéndit in barbam, barbam Aaron.
Like the precious ointment on the head, that ran down upon the beard, the beard of Aaron,
3 Quod descéndit in oram vestiménti ejus: * sicut ros Hermon, qui descéndit in montem Sion.
which ran down to the skirt of his garment: As the dew of Hermon, which descends upon
4 Quóniam illic mandávit Dóminus benedictiónem, * et vitam usque in sæculum
For there the Lord has commanded blessing, and life for evermore.
The psalm is frequently seen by the Fathers, as particularly addressed to religious. Cassiodorus, however, asserts its universal relevance:
"After the most holy preaching of the previous psalm, the prophet is now perched on the fourteenth step, and proclaims to the people blessed unity, urging those who bind themselves to the Christian religion to persevere in the one accord of charity.
Though some have opined that this message is to be addressed to monks, my view is that it is relevant to the harmony of Christians at large, for it is proclaimed not only to monasteries but to the whole Church, gathering into unity by a spiritual trumpet-blast all Christ's soldiers throughout the world. I do not dispute that it is addressed to saintly monasteries, but I believe that it should not be withdrawn from the general body.
So the place which embraced the gathering of the faithful people was worthy of honour; clearly that assembly was established before the Lord's incarnation gained it from the Gentiles by His precious blood."
The opening and closing lines make the psalm's subject matter clear, namely spiritual unity as a prerequisite for eternal life.
The two images in between, however, of oil flowing down the beard of a priest, and dew on Mt Hermon, require a little more work to explicate, and are dealt with in the verse by verse notes that start in the this post.