The dew of Mount Hermon. On the slopes of Mount Hermon, which is the highest peak in Palestine. Dew is necessary for life and growth. When plants are in the middle of a growing season water is so precious that dew is a nightly gift that God bestows upon plants. Here David is saying that when the tribes of Israel come together, and they are united, it it is like the dew of Mount Hermon was falling upon Mount Zion.
Unity is a great thing.
How good and pleasant it is that Central Presbyterian Church can sing Psalm 133 because God has blessed us for the past decade with a remarkable unity. People have come together in Jesus Christ. We don't have church fights. God has poured out his precious oil upon this church, the oil of His Holy Spirit. An oil that has brought us together and, like Aaron who was anointed for the priesthood by anointing with oil, so God has anointed each of you as a priest.