Psalm 133, the fourth of four psalms of David among the songs of ascent, concerns the joy of brotherly unity among God's people. As the second song of ascents in the fifth set of three, it looks in trust to God to provide His commanded blessing of eternal life. The key word in the psalm, missed in English because it is translated different ways, is the thrice--repeated yarad--rendered "running down" twice (verse 2) and "descending" once (verse 3). The idea is that goodness and blessing comes down from above--from God.
David may have composed the psalm when the tribes, after many years of conflict, agreed to unite under his leadership (2 Samuel 5:1-5 2 Samuel 5:1-5  Then came all the tribes of Israel to David to Hebron, and spoke, saying, Behold, we are your bone and your flesh.
 Also in time past, when Saul was king over us, you were he that led out and brought in Israel: and the LORD said to you, You shall feed my people Israel, and you shall be a captain over Israel.
 So all the elders of Israel came to the king to Hebron; and king David made a league with them in Hebron before the LORD: and they anointed David king over Israel.
 David was thirty years old when he began to reign, and he reigned forty years.
 In Hebron he reigned over Judah seven years and six months: and in Jerusalem he reigned thirty and three years over all Israel and Judah.
American King James Version×). As well, the psalm speaks to the pleasure of traveling together in harmony to keep the feasts in Jerusalem--and participating there in the great throngs of unified worship. Furthermore, the song applies to the blessing of unity within and among the congregations of God's people--even today.
The delightful unity described is between "brethren"--brothers--emphasizing family kinship. This goes far beyond immediate family. For people in Old Testament times this was understood in the sense of national brotherhood. And of course we in God's Church understand it to refer, on an even higher level, to spiritual brotherhood through the Holy Spirit of God. God's children getting along and happily working together is truly a delightful experience--and a blessing that comes down from Him.
The "precious oil" (verse 2) was the anointing oil specially prepared for use in the tabernacle (see Exodus 30:22-33 Exodus 30:22-33  Moreover the LORD spoke to Moses, saying,
 Take you also to you principal spices, of pure myrrh five hundred shekels, and of sweet cinnamon half so much, even two hundred and fifty shekels, and of sweet calamus two hundred and fifty shekels,
 And of cassia five hundred shekels, after the shekel of the sanctuary, and of oil olive an hin:
 And you shall make it an oil of holy ointment, an ointment compound after the are of the apothecary: it shall be an holy anointing oil.
 And you shall anoint the tabernacle of the congregation therewith, and the ark of the testimony,
 And the table and all his vessels, and the candlestick and his vessels, and the altar of incense,
 And the altar of burnt offering with all his vessels, and the laver and his foot.
 And you shall sanctify them, that they may be most holy: whatever touches them shall be holy.
 And you shall anoint Aaron and his sons, and consecrate them, that they may minister to me in the priest's office.
 And you shall speak to the children of Israel, saying, This shall be an holy anointing oil to me throughout your generations.
 On man's flesh shall it not be poured, neither shall you make any other like it, after the composition of it: it is holy, and it shall be holy to you.
 Whoever compounds any like it, or whoever puts any of it on a stranger, shall even be cut off from his people.
American King James Version×). "When the high priest was anointed, the oil ran down his beard to the front of his body and over his collar. This suggests that the oil 'bathed' the twelve precious stones that he wore on the breastplate over his heart, and this 'bathing' is a picture of spiritual unity" (Wiersbe, Be Exultant, note on verse 2).
In verse 3, Mount Hermon, a high, snowy peak on the border between Israel and Lebanon, was a significant source of water for Israel. One avenue for this was evaporation--carrying water from Hermon to settle in the south as dew and rain. Also, snowmelt sank into the Hermon region and emerged in many streams in northern Israel, even forming the headwaters of the Jordan River--the word Jordan also being derived from the word yarad. (Yaraden apparently meaning "coming down from Dan"--which was in the Hermon area). As water was carried this way from Hermon down through the land of Israel to the mountains of Zion, so was further evaporation and precipitation. Yet note that precipitation in general is not in mind here, but specifically dew. From around May to October, encompassing Pentecost and the Feast of Tabernacles, virtually no rain fell on Jerusalem, so that refreshment came only through the blessing of daily morning dew--and, the comparison is made, through the unity of God's people at His pilgrim feasts.
"The two similes (vv. 2-3) are well chosen; God's blessings flowed to Israel through the priestly ministrations at the sanctuary (Exodus 29:44-46 Exodus 29:44-46  And I will sanctify the tabernacle of the congregation, and the altar: I will sanctify also both Aaron and his sons, to minister to me in the priest's office.
 And I will dwell among the children of Israel, and will be their God.
 And they shall know that I am the LORD their God, that brought them forth out of the land of Egypt, that I may dwell among them: I am the LORD their God.
American King James Version×; Leviticus 9:22-24; Numbers 6:24-26 Numbers 6:24-26  The LORD bless you, and keep you:  The LORD make his face shine on you, and be gracious to you:  The LORD lift up his countenance on you, and give you peace.
American King James Version×)--epitomizing God's redemptive mercies--and through heaven's dew that sustained life in the fields--epitomizing God's providential mercies in the creation order" (Zondervan NIV Study Bible, note on verse 3). Moreover, anointing oil and water precipitation are both representative in Scripture of the Holy Spirit--sent down from God to transform and spiritually sustain His people.
This blessing from above is the source of the unity of God's people--as well as the closing blessing of this joyous life continuing through all eternity.