The Book of PsalmsPsalm 119

Ad Dominum. A prayer in tribulation. A gradual canticle. The following psalms, in number fifteen, are called gradual psalms, or canticles, from the word gradus, signifying steps, ascensions, or degrees: either because they were appointed to be sung on the fifteen steps, by which the people ascended to the temple: or, that in the singing of them the voice was to be raised by certain steps or ascensions: or, that they were to be sung by the people returning from their captivity and ascending to Jerusalem, which was seated amongst mountains. The holy fathers, in a mystical sense, understand these steps, or ascensions, of the degrees by which Christians spiritually ascend to virtue and perfection; and to the true temple of God in the heavenly Jerusalem.

The Douay-Rheims numbers the Psalms as the Latin Vulgate does. In Bibles translated from the Hebrew this is Psalm 120. Read Psalm 120 in the WEBC.