This latin phrase can be translated into English to "who takes away the sins of the world". It is found in various versions of the Mass and Requiem in reference to God the son as an opening to various requests, such as...

Pie Jesu Qui tollis peccata mundi. Dona eis requiem. Agnus Dei Dona eis requiem sempiternam. (Merciful Jesus, who takes away the sins of the world. Grant them rest, Lamb of God. Grant them everlasting rest.)

Naturally, it's found in the slew of compositions based around the words of the Mass. While learning Mozart's "Great Mass in C Minor", our conductor instructed us to pronounce the phrase as would The Count from Sesame Street.

If I may improvise <coughs>:
Qvi tohlihs peycaaarta mMHUUUUUUUNNnnnndiii (Ah-Ah-Ah)