John Tavener composed this specifically for Björk. She chants a prayer from the Eastern Orthodox Christian tradition, “Prayer of The Heart”, accompanied by the Brodsky Quartet.

“Prayer of The Heart” is a short, simple prayer that has been widely used, taught and discussed throughout the history of Eastern Orthodox Christianity. It is, for the Orthodox, one of the most profound and mystical prayers, and is often repeated endlessly as part of personal ascetic practice. It is particularly used in the practice of the spiritual life known as hesychasm. Based on Christ’s injunction in the Gospel of Matthew, “when thou prayest, enter into thy closet, and when thou hast shut thy door, pray”, hesychasm in tradition has been the process of retiring inward by ceasing to register the senses, in order to achieve an experiential knowledge of God. The prayer is particularly esteemed by the spiritual fathers of this tradition as a method of opening the heart.

Greek: Κύριε Ἰησοῦ Χριστέ, Υἱὲ τοῦ Θεοῦ, ἐλέησόν με (τὸν ἁμαρτωλόν).
English: Lord Jesus Christ, Son of God, have mercy on me (a sinner).

More info from Wikipedia HERE, and from Orthodox Wiki HERE.