When Aaron was anointed as Israel's High Priest they poured oil on his head. Anointing with oil was a symbol of the Holy Spirit. It was a high and holy occasion - not only for Aaron, but for all of Israel.
When God's people dwell together in unity it is good and pleasant, but it's as wonderful as when Aaron was anointed as the High Priest.
Here are some verses about "unity" from the New Testament.