Pange Lingua Gloriosi Corporis Mysterium is a Latin hymn written by Saint Thomas Aquinas (1225–1274) for the Feast of Corpus Christi .
It is also sung on Maundy Thursday, during the procession from the church to the place where the Blessed Sacrament is kept until Good Friday.
The last two stanzas, called separately Tantum Ergo, are sung at Benediction of the Blessed Sacrament.
The hymn expresses the doctrine that the bread and wine are changed into the Body and Blood of Christ during the celebration of the Eucharist in a process called transubstantiation.
The preceding gregorian chant is by the Capella Gregoriana. As it is on YouTube, I guess I can use it on this website also.
Only the first verse is sung as introduction.
Pablo Bruna wrote a simple and short, yet beautiful and rythmically original piece on it.