What is happening here
As the assembly prepares to hear the Gospel, they stand and sing the Gospel Acclamation — almost always "Alleluia, Alleluia, Alleluia" with a verse drawn from Scripture. (During Lent, when the joyful "Alleluia" is set aside, an alternative acclamation such as "Praise to you, Lord Jesus Christ, King of endless glory" is used instead.)
The Gospel Acclamation is a distinct liturgical act, not just a transitional moment. The Church teaches that Christ is uniquely present in the proclamation of the Gospel — present in the Eucharistic elements supremely, but present also in his Word, especially the Gospel which is his own teaching. The assembly's standing posture, the ministers' procession with the Book of the Gospels, the use of incense on solemnities, and the kissing of the book by the deacon or priest after the proclamation all signal this presence. The "Alleluia" is the assembly's welcome to the Christ who is about to speak.
The verse sung after the threefold "Alleluia" is taken from the Gospel itself or from another scriptural text that introduces the Gospel's theme. It is meant to focus the assembly's attention before the proclamation begins. In well-prepared Masses, the Acclamation is a moment of musical and spiritual concentration — a brief but real lifting of the heart toward the Word that is coming.
In the Tridentine Mass and in much of Catholic devotional history, this moment was also marked by the priest praying the Munda cor meum — "Cleanse my heart and my lips, almighty God, that I may worthily proclaim your holy Gospel." The practice continues in the current Roman Rite, prayed quietly by the deacon or priest before he proclaims.