Catechism of the Catholic Church
“‘Daily’ (epiousios) occurs nowhere else in the New Testament. Taken in a temporal sense, this word is a pedagogical repetition of ‘this day,’ to confirm us in trust ‘without reservation.’ Taken in the qualitative sense, it signifies what is necessary for life, and more broadly every good thing sufficient for subsistence. Taken literally (epi-ousios: ‘super-essential’), it refers directly to the Bread of Life, the Body of Christ, the ‘medicine of immortality,’ without which we have no life within us. Finally in this connection, its heavenly meaning is evident: ‘this day’ is the Day of the Lord, the day of the feast of the kingdom, anticipated in the Eucharist that is already the foretaste of the kingdom to come. For this reason it is fitting for the Eucharistic liturgy to be celebrated each day.”
Source: Catechism of the Catholic Church #2837