When Christ ascended into Heaven, He told his apostles that He would send His Holy Spirit and so they prayed for the coming of the Spirit. The Novena to the Holy Spirit recalls this period of waiting. It is an expression of the desire of the faithful to receive the gifts of the Holy Spirit which are enumerated in Isaiah 11 (Wisdom, Understanding, Counsel, Fortitude, Knowledge, Piety, Fear of the Lord). The modern version that you can download here includes reflections from Pope Francis.