Alleluia is the Greek transliteration of the Hebrew word that means “Praise the Lord” (Hallelujah). Martin Luther called the alleluia “the perpetual voice of the church” because it’s how the people of God have always expressed their joy when receiving His Word. It is part of the liturgical structure that Christianity inherited from Judaism. (Psalm 104ff, Revelation 19:1-6)
We stand when we sing the alleluia because we acknowledge that singing it marks the forthcoming climax of the Service of the Word, the reading of the Holy Gospel. However, it is actually sung in response to the Epistle reading. The Church praises the Lord because He has given us His Word.
The alleluia is omitted during the penitential season of Lent. An appropriate Lenten verse is sung in its place.