It is a Latin phrase meaning “In You, Lord”. In = In, Te = You, Domine = Lord. Also of note is that “Gloria”, the title of the song, is the Latin word for “Glory”. [AC] In Latin, the beginning of Psalm 31 is “In te Domine speravi…” “In you, O Lord; I take refuge.” As for the other Latin bits in “Gloria,” “exultate” and “miserere” — these are the beginnings of Psalms 33 and 51, respectively: “Exultate iusti in Domino…” “Exult, you just, in the Lord.” And “Miserere mei Deus…” is “Have mercy on me, O God…