From today's service: "O Holy Immortal, the Spirit, the Comforter, who proceeds from the Father and rests in the Son!"
What does the bolded mean?
That's a great question, I think, and seems to me an example of how the Church has preserved many ancient traditions through her liturgies.
This tradition of the Holy Spirit finding its rest in our Lord Jesus Christ has a Scriptural basis and seems to have been taken up particularly among the early Jewish Christians. There are profound similarities with the Pentateuchal treatments of the Glory of the Lord in the tabernacle, through the Sapiential traditions of Wisdom in search of a resting place, to the New Testament concept of the body of Christ as God's temple/tabernacle (and thus the resting place of the Holy Spirit). And since the Church is now the Body of Christ, she is the resting place where the Holy Spirit tabernacles ...
For anybody who is interested, below are some relevant Scriptural quotes, and at the end there is a quote from Jerome in which he preserves a very pertinent Jewish-Christian tradition:
Exodus 24:15-16: Then Moses went up into the mountain, and a cloud covered the mountain. Now the glory of the Lord rested on Mount Sinai, and the cloud covered it six days. And on the seventh day He called to Moses out of the midst of the cloud.
Numbers 9:22: Whether it was two days, or a month, or a longer time, that the cloud continued over the tabernacle, resting upon it, the Israelites would remain in camp and would not set out; but when it lifted they would set out.
Isaiah 11:1-2: A shoot shall come out from the stump of Jesse, and a branch shall grow out of his roots. The Spirit of the LORD shall rest on him, the spirit of wisdom and understanding, the spirit of counsel and might, the spirit of knowledge and the fear of the LORD.
Isaiah 66:1: Thus says the LORD: Heaven is my throne and the earth is my footstool; what is the house that you would build for me, and what is my resting place?
Sirach 24:7-11: [Wisdom:] Among all these I sought a resting place; in whose territory should I abide? "Then the Creator of all things gave me a command, and my Creator chose the place for my tent. He said, 'Make your dwelling in Jacob, and in Israel receive your inheritance.' Before the ages, in the beginning, he created me, and for all the ages I shall not cease to be. In the holy tent I ministered before him, and so I was established in Zion. Thus in the beloved city he gave me a resting place, and in Jerusalem was my domain.
John 1:14: The Word became flesh and lived [literally, "tabernacled"] among us, and we have seen his glory, the glory of a father's only son, full of grace and truth.
John 1:32-33: John testified, 'I saw the Spirit descending from heaven like a dove, and it remained on him. I myself did not know him, but the one who sent me to baptize with water said to me, 'He on whom you see the Spirit descend and remain [Gr. menon, lit., "live/dwell/lodge] is the one who baptizes with the Holy Spirit.'
John 2:19-21: Jesus answered them, "Destroy this temple, and in three days I will raise it up." The Jews then said, "This temple has been under construction for forty-six years, and will you raise it up in three days?" But he was speaking of the temple of his body.
1 Peter 4:14: If you are reviled for the name of Christ, you are blessed, because the spirit of glory, which is the Spirit of God, is resting on you.
Gospel of the Hebrews (via Jerome): Further in the Gospel which we mentioned above we find that the following is written: It happened when the Lord ascended from the water, that the whole fountain of the Holy Spirit descended and rested upon him and said to him: My son, I expected you among all the prophets that you should come and that I should rest upon you: For you are my rest, you are my firstborn son, who shall reign in eternity. (Jerome, Comm. Isa.)